People sometimes don't know that certain topics or manners of speech are offensive. I, for one, see sexual topics as not much more interesting or taboo than talking about digestion or elimination or any other bodily process; through experience, of course, I have learned that other people do not wish to talk about them.
Or maybe it's just a matter of experimentation: People learn that if you perform certain actions, you get certain results; and they have learned that on an Internet forum, posting what is considered offensive results in a lot of responses. It reminds me of a rat in a Skinner box: Push the button, get a food pellet; push the button, get a food pellet... it's a matter of positive reinforcement. Perhaps these people simply enjoy the fact that they are creating a stir.
In either case, it really isn't very prudent to expend emotional and mental energy on an anonymous person posting something offensive on an Internet forum. You don't know them; they don't know you; so how could any perceived insult or embarassment be at all personal?