depends on your definiiton of cheating, if your girl has cheated on u she will do it again as she is obviously motivated by power, money or looks and not love.
1. Slippery slope fallacy. 2. It depends primarily on the reason for cheating. If the cheater cheated because of some deeply ingrained complex fueled by insecurity, then most likely the cheater will cheat again. If the situation was a really unique, awkward situation where all the bad things happened at the wrong time, or through a moment of deep emotional crisis, then the cheating will likely be seen simply as a growing experience and is less likely to happen again.
I have cheated on almost every one of my girl friends when i was 15 and 16 but now that I'm older i realize that it's stupid and to just stick with one or two