Oh yes, my first GF did this as well, and in retrospect there's no surprise: She was a cute athletic brunette and I was foolish about relationships whereas she had probably been pumped and dumped several times before. She was sneaky about it at first but later got more open about it as well. For example, at one point it turned out she had been fucking a drug dealer who already had two baby mamas (this guy was a whigger by the way, not black, but she may have fucked blacks too for all I know). When I confronted her about it she was like "relax, I'm not even attracted to him, I just wanted free drugs." Really an eye opener, that was.
Actually in retrospect, I should have appreciated that situation more than I did. She wanted to go to strip clubs and hit on girls in tandem, or find girls for us to have threesomes with. I never really took near as much advantage of what I had going on at that time because then I actually believed in commitment and so on. And when you're young you think the opportunities are limitless...