Well, here's my two cents...
The MPAA seems to be heavily influenced by the mass public, especially adults over 35 or so. Many of them are parents of small children, and like it or not, explicit sexual content is often seen as the "greater of two evils", violence being the other in most cases. Parents at large are (seemingly) more willing to have the film industry educate their kids about violence, rather than teach them unhealthy attitudes about sex. My personal opinion is that the MPAA should judge both, not just one of them. That's one of the many reasons why ratings often have a content description added to them.
Example: "R - Restricted, for language, violence, disturbing images, and overt sexuality."
the sad fact is that many people don't bother paying attention to those descriptions. they think, "PG-13, and my kid is 16...no problem", instead of taking the content into account and being responsible for what gets into their kids through the silver screen. It boils down to a moral debate about whether or not certain content is truly acceptable, and since everyone has a different opinion...well, the point is that it is the job of parents as well as the media to safeguard this generation from decline, and at the moment, only about a third of either group are taking that responsibility seriously.