Well, if I'm dating someone I care about whether he's dating someone else. Especially if the relationship is potentially getting physical. Some people have no problem dating several people at once. But for me, I'm unlikely to be interested in dating someone who's already involved with someone else. There is potential to get hurt, and for things to get complicated. It doesn't appeal to me. I don't like the idea of smooching with someone who's probably done the same thing with someone else the day before, eeek!