Think it's mostly religious shit. Someone went around tellin' people God said sexual urges are corrupt and givin' into 'em is sinful unless you're doin' it specifically to have kids with someone you're married to. Now the idea's so stuck that even people who don't give a shit about God think there's somethin' bad or shameful about it and they're doin' something wrong if they show the parts to anyone but the person they're datin'.

Don't help that the messed up attitude means most people who see that much skin think it's an invitation even when it ain't.