It happened to me one day that 2 different girls told me in the same day that they wanted to be my girlfriend. They didn't know each other, so i used this to my advantage and dated them simultaneously for about 2 weeks, trying to decide which one i liked best.
Eventually, it didn't click with one of them and we broke up, but i continued my happy relationship with the other girl and all was fine. Had they found out about this it maybe would have been a different story.
However, my point is, in the same way as i have dated 2 girls in the same time, so did missyousomuch's girlfriend. She dated 2 guys in order to decide who she liked best.
I don't call that cheating. It's more like a competition between 2 guys for the same girl. I would be proud to find out that my girlfriend has been comparing with some other dude in the same time, but she chose me in the end. This, of course, assumes that the girl is indeed done with the other guy and she becomes exclusive with me. But can she be trusted?.. hard to say...
I just brought another point of view to the table: that of a competition between 2 guys and one is chosen, hence he's a winner. It doesnt mean that the girl will keep dating other guys after that. Just my 2c.