so birth control is causing women to have sex with more dudes, making it less likely that women have sex with ugly/weird dudes? 
actually yes, that's how the theory goes. it's a little counter-intuitive on its face but if you think about it, you'll get it.
1. monogamous relationships cause more egalitarian sex numbers.
2. the pill leads to fewer monogamous relationships.
3. the pill leads to less egalitarian sex numbers.
if women get to be choosier, then they're not going to choose uggos/weirds.