In the media, sex is often portrayed as "cool". If you're not having sex, then you're a sniveling misfit and a loser.
Sex is guaranteed to sell a product, or to bump up the ratings and views on a movie or a television show.
A sure-fire way for a singer or an actor to market themselves is to "sex themselves up" with makeup, styled hair, a fit body (or plastic surgery to make their faces and bodies seem more "attractive"), and scant clothing. Whether this works is dubious, but common wisdom insists it does.
Sex is an innermost desire for most human beings. Enjoying it and wanting to have it are perfectly normal. But this is one thing that the media may have taken a little far.
Characters who are portrayed as popular, well liked people are almost always hinted to be or described as sexually active, especially if one of them is the lead. Their sexual prowess is part of the reason they're "cool" in the eyes of the other characters and of the viewers, or at least that's the theory. If a whole bunch of people want to do them, then that must make them "cool", right?
Not only is it a good way to give the character a swanky "edge", but it makes them seem more likeable and "mature" to many viewers. It's important for Fanservice.
These people are often self-absorbed arrogant bullies who rub their powers of sexual prowess in the faces of anyone who isn't as popular as they. They spend their entire lives going through lovers like pairs of socks because it makes them "cool".
Often this projects a false message to impressionable young people and leads them to believe that having sex will make you happy and solve all your problems in life (because, of course, the only thing you should ever consider a problem should be that you're not having sex), or it will make you more "mature", or will gain you a lot of friends. Unfortunately, Real Life generally does not work that way, and people have a tendency to find out about that the hard way (heh). Not to mention that the media often project this message alongside its exact counter, leading to more confusion.
Sadly, there are people who behave as though this trope is true in the real world. Lots of them.