As much as we'd like to deny it sex is a large part of our culture. From the moment we know babies exist we're trying to figure out where they come from, how they were made. At any age sex is interesting. It's always changing. The truth is pop music that talk about sex is really just a reflection of how society views sex. In the 50's it was something to avoid talking about but it was hinted at and music then reflected that idea. Now sex is normal, widely talked about and considered fun so our music reflects that idea about sex.
It seems to have increased with the video age - When I was young a video was the band performing a song then they started having little stories now they all seem to be about semi clad women. Most of the stuff they wear in videos today would make a stripper blush.
Because music like that is meant to be marketed, not to be creative. These big music record labels know that sex will sell, so they'll incorporate as much of that as possible into their mediocre craft.