Just gonna throw this out there: When males get sexually aroused, their penises get hard, and when females get sexually aroused, their nipples get hard.  It's not quite a like-for-like similarity, but a woman's vagina cannot get erect (it, well, gets wet), and so you can't see its, uh, hardening.  But just like a man's bulge stretching his pants, a woman can show her randy mindset (or at least that they're cold) by wearing a tight shirt.  (It helps, I guess, if a woman isn't wearing a bra, either; a woman not wearing a bra out in public is most likely tipping off that her mindset is already pretty randy).

I am thinking this because I'm looking at Zishy a lot (it's at zishy dot com).  Every other day there is a new pictorial featuring a woman who likely ends up getting naked in a private space.  But the spread usually starts off with her flashing in public, and one of the first preview photos (you'll have to subscribe to see the whole roll) usually is a shot of her facing the camera with her top on, but with her nips poking through.  Because, you see, she's excited to show her breasts pretty soon.

God, I want to bite into a woman's hard nipples so hard.