If you look on Freida, it says that of the 3,002 residents/fellows, the average # of females is 8.5%. On the interview trail, I would say the applicant pool approximates that number. At some interviews there were a lot of women (like 4-6), whereas, at other programs there was like one.
I don't think any programs go out and advertise that they're anti-female, but at some interviews I've heard the residents say that the program won't take a woman, because they've had bad experiences with them in the past. I would say, watch out for programs that have had women quit on them, that probably makes it tougher for a woman to get in. You can't really figure out which programs have had women with bad personalities though.
Some programs that I've been to that might be female "friendly" are UT Southwestern which has like 5 female residents, Case Western has like 4, Northwestern has 3, and Stanford has a couple. I'm sure there's more programs out there with multiple female residents too.