Richard Feynman's sexism was the subject of a recent defense in Scientific American, which prompted several rebuttals. Among other things, people defend Feynman by saying he was a "product of his time".
I'm skeptical that being a man in the 1950s necessarily meant you were sexist in the way that Feynman was. If lots of men were "negging" in Feynman's time, then why would he have thought it was an interesting anecdote to tell? Feynman himself did not depict his pickup practices as products of his time, but rather products of his own cleverness.
I would go further than that. In general, I believe that even when people truly are products of their time, they should still be judged by today's standards, no matter how anachronistic. I believe this because I am a moral pragmatist, and also because I don't believe in the afterlife.
As a pragmatist, I believe the function of "moral judgment" is to encourage better behavior. And yet moral judgment has limited power to change people. Sometimes there's an powerful underlying "reason" why people do bad things. In these cases, moral judgment may be unwarranted, since it will just make people feel bad about themselves without doing anything to change the underlying reason for their behavior. And yes, I consider it an intrinsic evil to make people feel bad about themselves, no matter who they are.
So when people are a "product of their culture", this can be a mitigating factor to their moral responsibility. We can't expect people to deviate significantly from their surrounding culture, so sweeping moral judgments may be a waste. (On the other hand, moral judgment is also an excellent tool for changing the culture, so there's that.)
This framework for viewing morality has some odd consequences when we look at past people and past societies, especially when the people are all dead, and the societies gone. What is the point of judging these people at all, if their behaviors are set in stone? It doesn't do any of those dead people any good. Since there's no afterlife, it doesn't even make the dead people feel good or bad about themselves.
Therefore, the function of morally judging people in the past is not to change the past, but to guide the present. As such, the moral standards of today are the only moral standards that are relevant.
When we say that what Feynman did was morally acceptable, there are two messages we could be sending. The first message is the direct message: sexism is acceptable. The second message is more indirect and abstract: we're saying that if we live in a society that has sexism, then it is acceptable to assent to the sexism, and even participate in it. Since we do live in a society that has sexism in it, the second message also basically amounts to saying sexism is acceptable. Neither of these messages seem appropriate to me.
Another example is that we know Darwin was racist. My understanding is that Darwin was less racist than many of his contemporaries, but he also lived in a colonial society which thought of other peoples as savages, or somehow lesser. If we were to defend Darwin's racism, there are two messages we could be sending. The first message is that racism is acceptable. The second message is that if we live in a society that has racism, then it is good to reject that racism, even if only in baby steps. I think the first message here is a bad one, while the second message is a good one. Therefore, I have more mixed feelings about Darwin than I do about Feynman.
This analysis applies to people and societies in the distant past, but not necessarily to societies in the recent past, nor to foreign societies in the present.