Here's why women want you when you're taken:

You're taken.

Some women see a man who is taken, and think to themselves,

"Hey.. this guy has a girlfriend. There must be something good about him worth having. I think I'll attempt to make a move."

Not all women are like this, but some women, either conciously or subconciously, think like this. Some women want the challenge that going after a man who is taken brings. Some women are just dysfunctional.

There's a reason people say "all the good ones are taken". People covet what they can't have.