I think the issue is impressionability. Death and killing is a black and white topic. You don't kill, killing is wrong. We know this. We know that no matter how many people we kill in a video game, and how brutally, that we should never do it in real life.
Sex, however, is something that is open, available, and more open to moral interpretation. Nudity is often seen as objectifying women, and/or sexually enticing, and it is through constant moral conditioning that teens are instructed by their strict parentage that sex before marriage is wrong, and it still fails, and anything that is seen to promote sexual acts is therefore adding fuel to the fire.
So, basically, killing is bad. Sex is arguably bad, and therefore at bigger threat for compromising a young player's impressionable morals.
Sex, however, is something that is open, available, and more open to moral interpretation. Nudity is often seen as objectifying women, and/or sexually enticing, and it is through constant moral conditioning that teens are instructed by their strict parentage that sex before marriage is wrong, and it still fails, and anything that is seen to promote sexual acts is therefore adding fuel to the fire.
So, basically, killing is bad. Sex is arguably bad, and therefore at bigger threat for compromising a young player's impressionable morals.