I think the reason is the current crop of heterosexual directors don't have any balls.
When people talk about that, they're generally referring to American made films. Foreign films don't have a problem with female nudity.
There are probably a lot of reasons, you're probably right about the "directors not having any balls", but there are other reasons. The internet has now made female nudity redundant and it's not a box office draw anymore when you can easily see it on line (and fewer actresses are willing to do it because of that). Also maybe because of proliferation of porn, female nudity seems to be out of style now. It seems to be stigmitized as "dirty, exploitive, and disrespectful of women". Male nudity is usually presented as "comical", which is more acceptable than the "pornogrpahic sexy nudity" usually done by women. I don't really think it's a feminist/gay agenda, but there does seem to be a move towards film makers targeting females and the gay auidences with their nude scenes, and they don't have "the balls" to challenge social norms that female nudity is not pornographic or dirty (and they're afraid of offending their audience).