Mdtryhht

(edited this answer to consider some points raised in the comments and try to address OP more constructively)

Yes, it is acceptable for a professor to work to counter the effects of gender discrimination.

Yes, it is acceptable for a professor to warn a male-predominant class to not discriminate against women by giving them the "easy" tasks.

The professor is not accusing you as an individual of sexism, they are pointing out researched ways in which women are discriminated against, especially in STEM areas where women are under-represented.

Although your initial reaction was that this was a waste of time, I think you can use this experience and the feedback you get here as an opportunity to reflect - it seems from your comments that you are open to this.

I'd suggest you try the implicit bias test from Harvard, I was surprised by how much implicit bias it showed that I have. Some research has shown that people who take this sort of test are more introspective about their own biases afterwards. Even if you show little bias on that test, it may be instructive to see how much bias is seen in the population as a whole because people tend to underestimate bias experienced by other groups.

I don't think having implicit bias is something that reflects poorly on any individual, it's more of a product of society. What reflects on individuals is whether they try to become aware of the biases experienced by others and how their own implicit biases might influence their behavior.

I think your professor was trying to point out some of these biases to make students in the class more aware of them. It seems unlikely to me that any men in the course would literally think to themselves "Let's give the women the easy/secretarial tasks in group work!" (and if they do, they are probably never going to be convinced otherwise) - instead, they might accidentally do it by simply not thinking. By simply thinking about it more, and even by asking this question here, it seems to me like your professor was successful, even if you ultimately decide that you disagree with the approach.

Carter, A. J., Croft, A., Lukas, D., & Sandstrom, G. M. (2018). Women’s visibility in academic seminars: Women ask fewer questions than men. PloS one, 13(9), e0202743.

Cheryan, S., Ziegler, S. A., Montoya, A. K., & Jiang, L. (2017). Why are some STEM fields more gender balanced than others?. Psychological Bulletin, 143(1), 1.

Kaatz, A., Gutierrez, B., & Carnes, M. (2014). Threats to objectivity in peer review: the case of gender. Trends in pharmacological sciences, 35(8), 371-373.

Lerback, J., & Hanson, B. (2017). Journals invite too few women to referee. Nature News, 541(7638), 455.

Monteith, M. J., Ashburn-Nardo, L., Voils, C. I., & Czopp, A. M. (2002). Putting the brakes on prejudice: On the development and operation of cues for control. Journal of personality and social psychology, 83(5), 1029.

Swim, J. K., & Sanna, L. J. (1996). He's skilled, she's lucky: A meta-analysis of observers' attributions for women's and men's successes and failures. Personality and Social Psychology Bulletin, 22(5), 507-519.

Is my view justified?

It sounds like you feel disrespected by the professor’s implicit assumption that some members of your gender engage in sexist behaviors and need to be told not to engage in them any more. And yet, you yourself acknowledge that this assumption is likely correct. Her statement was of a general nature; she didn’t personally accuse you of anything, so personally I don’t see why you would feel offended.

You are perhaps justified in thinking the discussion was not the best use of the class time, but that would be your opinion, which may differ from other people’s opinion. Not all students have to agree all the time that what a professor is talking about at any given moment is the best thing for them to talk about. Some disagreement on such things is perfectly reasonable and probably unavoidable. But just the fact you think something was a waste of time does not imply that the professor is doing something wrong by talking about it.

Is such behavior acceptable for a professor?

Yes. The professor is doing their job of trying to achieve the best educational outcome for their class. Whether or not I agree that this topic was worth bringing up in class in the way they brought it up, there is nothing about it that qualifies as “unacceptable” behavior, such as harassment or discrimination, as far as I can tell.

Should I let the professor know the way I feel?

You are free to do so, but it’s not clear to me what you are hoping to achieve by doing that. I can’t advise you on whether you should or shouldn’t do it.

No, it’s not acceptable. Discrimination against men is just as unacceptable as discrimination against women, and, moreover, the message that she was sending to the women in the class was essentially “Prepare to get discriminated against,” which isn’t a good message to send.

What should you do about it? Talking to her is unlikely to have an effect, because it is very difficult to convince people to alter their political ideologies, and she might have tenure so she might not have anything to fear from you making a complaint to the university.

Instead, just give her a poor rating in your feedback survey for the course, and leave a comment in the general feedback comment section explaining why. Universities use these surveys as a way of evaluating the performance of their instructors, so a poor review may well hit her where it hurts: in the wallet.

Is such behavior acceptable for a professor?

Yes. This the whole idea of academic freedom (as exemplified by tenure) -- professors should be able to work with uncomfortable issues without worrying about getting fired. I recognize that academic freedom would likely not protect them if they had made similar but politically-incorrect statements, but that's a separate issue.

Should I let the professor know the way I feel?

Depends on the professor. If they are a good professor, there might be an opportunity for discussion and perhaps you would both learn something. But sadly, many professors view this matter as sacrosanct, and will just write you off as a misogynist without allowing an open discussion. (In their defense, many students also broach the matter in a very hostile way..."you hate men" sounds very different than "Some of your comments made me uncomfortable.")

Is my view justified?

Doesn't matter. This is a well-studied, very controversial subject where tempers run hot and everyone thinks they are an expert. Studying such subjects has its place, but I'm assuming you are not currently an expert in gender relations. You're certainly entitled to tell the professor how their comments made you feel, but rather than getting into a debate with your professor, I would suggest you focus on learning STEM.

I don't really see this as some version of reverse discrimination or sexism against the men in the class, STEM fields have been dealing with a gender diversity issue for years now and efforts to curb this are only in their early stages. As someone who is currently pursuing an advanced degree in a STEM field, I tend to be optimistic and say the issues of sexism and discrimination in the sciences are (at least now) primarily implicit and unintentional, but that arguably makes this statement from your professor even more important. It may not have been intended to presume a discriminatory worldview, but instead to assuage any unconscious bias by trying to make students aware of these issues (which again, are salient in STEM fields right now).

I would also say the professor may have thought the warning more necessary given the proportions of the class. To quickly address your reasons point-by-point:
1. This does address group membership, but there's no way to treat it individually in this environment and it's still an important issue.
2. This is kind of true, but only because the fields have been male dominated in the past, which is what makes the statements necessary in the first place.
3. Could have been sexism, but I'd give the professor the benefit of the doubt and think of it more as just an attempt to encourage awareness of this potential issue.
4. This is a STEM class, therefore the diversity of the STEM fields are a pertinent topic, it may not have been a discussion of Newton's Laws, but that doesn't make it unrelated to the field. Plus, what's 5 or 10 minutes over a semester?

Unless your university is very far from the standard in the field, the offense you had to endure by feeling blanketly targetted because of your gender will be minuscule to the offense a female in a STEM career will have to endure not once but on a recurring basis.

I've been in engineering with about 3% of female students. Of those, the majority had a father in a STEM field and no male siblings providing an outlet for the father's role modeling that would have talked down their ambitions.

That's probably the situation from one generation before yours and society's clichés and images (the stuff feeding TV tropes) don't change all that fast and prejudices tend to double down before they move. I've seen my fair share of mansplaining in a number of disciplines including less male-dominated ones, and I've delivered my fair share of it as well: falling into the respective patterns is really easy.

You feel belittled by this advice. Would you also feel belittled by the suggestion to electricians to always keep one hand in the pocket when poking around live circuitry with a screw driver or probe? It is also an advice to behave in a manner minimising the results of stupidity that intelligent people should not be prone to in the first place.

I've brushed stuff in a TV set while pointing a grounded lamp with the other hand. I've seen a physics professor demonstrate the effectivity of a large Faraday cage from the inside while holding a wired microphone.

If you think yourself proof against stupid shit, it's probably because you haven't had enough opportunity to see yourself in the mirror.

Telling your offense to your professor will prime her to put your gender-related behavior under her scrutiny. You have nothing to gain from that.

Maybe wait out a year and keep your eyes and mind open. If you then still think that all-in-all her advice was unwarranted, reconsider telling her.

This sort of an approach is becoming increasingly popular as a way to fix the gender imbalance in academia. Gender discrimination is a problem that exists almost everywhere in society, including in academia. So, this is obviously a problem that needs to be addressed. The professor cannot be criticized for addressing this issue in class, but one may question if this is going to be effective. If the professor is concerned about social dynamics among students affecting female students adversely, then that suggests that one should not organize group work where such effects can manifest itself and compromise academic results.

The bigger picture is that Western countries where efforts to curb gender discrimination have led to a lot of success when it comes to getting women in leading positions in industry, still perform quite poorly when it comes to curbing the gender imbalance in academia. If we look at other countries, we see that Iran outperforms most Western countries when it comes to getting women into academia. If we look at the former Soviet Union, we see that the participation of women in science there was an order of magnitude better than today in the West. Clearly these results cannot be explained from a model that assumes that gender discrimination is the most relevant factor.

I.m.o., the reason why today we have such a huge gender imbalance in science is because we've organized science within a capitalist framework. This has dragged down the entire scientific enterprise, there are far less women and men at work in science than there should be. Women are affected more than men by the way we've organized science, but all we notice is the difference between the participation of men and women, which is just the tip of the iceberg.

If society makes more room for science by making available more funds, then this will lead to more jobs in science. Scientists will have better job security, this will go a long way to get far more women (and also men) into science and keep them there. Women are then affected more by these issues because long term job security tends to be more important for women than for men in determining career choices. Also gender discrimination will have a far greater effect when job security issues are also at play.