Its a matter of seducing them, I imagine. I have been messing with a lot of women in the last year who tell me, "Oh, by the way I have a boyfriend", "oh Im seeing someone right now". This my friends is a self-defense mechanism that they always throw at you. You cannot approach it directly, and must ignore it at all cost.
My appraoch is "thats your problem, not mine", or "good thing I dont have one of those", "Im not the jealous type so you can keep him". These girls are throwing themselves at you and try to defend at the same time, feminine logic which is to never be studied and not in the realm of human knowledge.
In my solemn opinion, if a woman is doing this she is confused and ready to be taken advantage of. her boyfriend is the least of my concerns, and I don't have to deal with it (if forced to I will). I seduce, she throws herself at me, its fair game. If she truly likes the other man, than she will run away, if not well its obvious what to do.
I could go on this subject for a while, even though I am young, I have put up with these situations enough recently to know my share of it.