Just like Red said that they do it since either they just learned the birds and the bees, or they are making a joke out of it. What some people don't understand is how -whats the right word- disturbing it can be to others. This was one of the reasons FeralHeart was shut down for an entire week, rules made stricter, and bans lengthier. When people do that the best thing to do, even though it may sound gross to you, is take a screenshot, and write down the users' names. Then from the forum, message a admin, and attach the screenshot and the names.
If there was a way to make buttswing into -> Tail wag, with the tail wagging like a dogs tail, and the butt moving the very least.