I was cheated upon by my lover at the time (she was 16 and I was 17, so that really nullifies the legitimacy in most peoples' eyes). It was when I was at my prime in terms of working out, and she was a cheerleader/bellydances (which was totally kickass). She didn't directly tell me she cheated, as she said she didn't want to get too attached to one person (after a year), which could be taken for truth, or could have been a blatant lie. I knew her long enough to know she did love me, though I could assume she probably started growing bored toward the end. But I digress. About a year and a half later, she sent me an email saying she was sorry, and that the person she left me for did the exact same thing to her. Being cheated on hurts, but it kind of comes with the territory of the whole thing, and especially in the high school and college age. If it goes against your morals, I'd suggest you don't do it, but it's more than likely going to happen at this age, so I'd say it's best to stick with flings, or closed casual relationships, but getting serious at this point and time is just asking for trouble.
Of course, everyone is different. Some people do find someone they stay with at this time, and these things can get complicated. Some of my older friends said they found someone special in college, and then got back together for good after that age, though it's not necessarily an excuse for stalking, and a horrible thing to lead people on with, if you don't actually follow through. But I digress.