Sometimes it just happens. You're in a girl you know is entered. The reason is that you should forget about it quickly, and you will remember how someone else used to give you a girl. Surely it was nothing pleasant and now you want to do the same? Do you want to avenge a man in some way?
Doesn't that just do it?
Everyone knows it's not done, but it still happens. In the end, a woman will often be seduced because her current acquaintance is already a long way to destruction. Then you have no reason to feel remorse. A girl who is emotionally lacking in a relationship may be flirating with you, but nothing more. Maybe we'd be surprised at how many satisfied long-term relationships arose because the boy had taken the girl over to someone else.