I think a more interesting question is: Why don't people dress sexier in real life (RL)? A reason for that might be that people are too afraid to. They don't want people thinking of them as, or calling them, a ***** for having the guts to wear something sexy. Also, people are less likely to protect you from sexual harassment if you're dressed provocatively in RL, because some people are stupid enough to think you're asking for it. Some people might also be ashamed or worried about their bodies in RL, too.
As for me, I tend to dress sexy in Second Life because it makes me feel sexy. Also, I like showing off my avatar and all the hard work I put into her. Though sexy is a relative concept as many things could be considered sexy. Take men in suits, for instance, or even women who are dressed more sharply. Why do you think the who schoolgirl look is somewhat popular?
That being said, I don't really have much unwanted attention from people. If you don't want someone bugging you in some way, you just have to ask nicely and they'll generally leave you be.