Interesting, and I agree with the posters above, who say that it is absolutely 100% morally reprehensible to allow someone to cheat with you (although I would say the cheater has a greater moral fault). However, I don't agree with those who have suggested that someone who does this is likely to cheat in a relationship, or do this again. You can never know, and I feel judging people on their past mistakes is unfair.
I have been the other woman, in a situation where I knew that my best friend had a girlfriend, and I knew the girlfriend (albeit not well). I was in love with him (a poor defence, but the only one I have) and he told me he loved me, and that his relationship was going really badly. We never went all the way, but began acting more like a couple, until he told me that he wasn't going to end his relationship. During that time, I felt very guilty vis a vis his girlfriend, but felt that if we were going to be a couple eventually it made little difference, so what I was doing was "ok". When I found out that actually we were not going to become a couple, I told him I no longer wanted anything to do with him and not to contact me again. In this situation, I would describe myself as entirely morally at fault, but I know that I would never ever do anything even resembling that again.