I spent two summers doing research in physics as an undergrad, with the NSF's REU programs. For one of them, I was the only woman out of eight students. This is about the national ratio of female physics undergrads to the total population, or was when I was an undergrad, and while I haven't followed Physics Today as obsessively, I believe it's similar now.
In all honesty, I was at least as intelligent and at least as talented and at least as enthusiastic about physics as any of the guys there, and obviously smarter(/etc.) than at least a few. But I spent all summer fighting the assumption that I was not as competent as they were, not just because I was a woman but because we all knew that institutions would go out of their way not to have single-gender programs. The default assumption was not that they took the top eight students who applied, but that they took the top seven students who applied, and then the top female applicant to round things out to fit the statistics. The rest of them could be there as physics students. Knowledge or assumption of a quota system made sure that I had to be there as a woman -- even if I'd been their top applicant of the year.
If speculative fiction magazines announced that they were having set-aside spots for women writers in their issues, I would get myself a genderless pseudonym if I didn't decide to stop submitting to those magazines completely. I will be damned if I'm ever going through that again if I can help it. And I wish it on no one else, either. It was absolutely miserable. A quota system means that your work always has to be twice as good as anybody else's to demonstrate that it was good enough to begin with. Hell with that. Categorically and absolutely: I am not. Going. Back.