That's America for you. I kid you not, go to the local mall ... watch some TV commercials, or anything open to the public and you'll notice some really lewd advertising of a woman showing off her body. But as long as no 'nipples' are exposed, they can show as much 'skin' as needed be to draw attention. Hell, chop em up gory movies can be PG-13 with intense violence slapped on there but the moment a movie shows 'nipples', suddenly it's Rated R. I think cutting someone's neck off with a chainsaw with blood flying off like a sprinkler is WAY more graphic & disturbing than seeing a woman's titties ....
Speaking of tits, most PG-13 movies can get away with nudity if they hide the nipples or crotch area. Yeah talk about bullcrap but that's how the American system works. There was a PG-13 movie with a fully nude women with her side boobs showing... but because no nipples were showing, they got away with the 3-4 second scene. (To be fair she immediately went to grab a blanket, in order to cover her front but ... you still see most of her body regardless). But why can men get away with showing nipples? Because our society is mostly ran by the male gender, even though it's still nonsense to let men have their shirts off and flex those pecks.
Men been going topless for centuries though, while we began to overthink about the female body and because 95% of the time women wear bras, we've allowed the sight of their nipples to be taboo ... in public of course. It's not really a sin but you get my drift.