Sex is outside the everyday experience of most people, particularly for children. If sex is not outside the daily experience of a child then that is a problem. Anyway the point is that repressing sexuality and glorifying violence results in some very weird social dynamics, many of them bad.
You may not have noticed but "want their child to do it later" isn't talking about the present.
That is not the point, you said you found it curious and odd not that you found it "bad". I only presented some reasons I consider plausible as to why it is like that not reasons for why that is good or bad. I don't have to think something is good to try to explain it.
Sex is only uncomfortable to talk about because they are told not to talk about it. There are plenty of places in the world that are much more sexually liberated than the US. Most sexual repression is religious in origin. Frankly if you aren't comfortable about having The Talk with your kids then you probably shouldn't have kids because you aren't mature enough yet. Most images and representations of sex are also "just fantasy" as you put it.
The reasons for it being uncomfortable is completely irrelevant to the simple fact that it is. We know it is because of the very thing being discussed - movie ratings consider sex worse than violence. Again whether that is a good thing or a bad thing is completely irrelevant to trying to understand why that is the case.
People who probably shouldn't have kids do have kids. And surely if they aren't mature enough to yet then they are precisely the group that cares about movie ratings since they need others to guide their decisions. Again it doesn't matter that they shouldn't have kids, the simple fact is that they do.
I defy you to find a movie or TV show or video game that shows a realistic portrayal of sexual activity in a non-ironic way. Young girls in particular get a very conflicted set of messages about sexuality. And if you think there is no violence in our country I refer you to the number of homicides and violent assaults that occur annually. You don't have to be a special forces soldier for violence to enter your life.
And they don't realistically portray violence either. Realism wasn't the point. Fantasy was being used not to describe the lack of realism in the movie but that such violence is unlikely to occur in the lives of the people who care about the ratings.
Most people have never been shot and have never shot someone. Most people have had some sort of sexual activity. For children replace "have" with "will". Or do you seriously think there are more homicides and violent assaults annually then there are sexual encounters?
I have *never* been involved in an incident of violence in the US. I've never hit anyone. I've never used a weapon against someone. No one has ever hit me. No one has ever used a weapon against me. I have caused two pregnancies while in the US. Every person I know has engaged in more sexual activity than they have violence. Yes, there are people who grow up in say a violent street gang or the military will see more violence than typical. But the typical is what matters for movie ratings.