One of the reasons I think violence is more accepted on TV than sex and nudity is violence is essentially black and white, you tell your kids easily, don't fight, don't hit etc. but sex is more gray. yes you don't want them having sex early, but sex itself isn't wrong, it requires more explaining and can become very complex an issue. to me that's why it's best to basically leave it out of kids properties. many of them know what mommy and daddy do, but I think even in same sex families, the kids don't know what daddy and daddy do (or mommy and mommy) they just know they have a matching set rather than one of each.
I may be wrong but again gay sex requires more explaination than hetero sex based on mechanics and in the beginning of conversations, purpose.