I'm going to have sex with a girl who cheats on her boyfriend.
Why would you have sex with this girl?
Everyone cheats. It's normal for people to sleep around, even if they are in a relationship.
I understand that sex is tempting, but doing something to encourage someone to cheat on their partner is wrong. You are ruining their relationship and hurting their partner by doing this. Their partner will be extremely hurt when they find out what is going on.
No she might not even find out. I'm going to have sex with her behind her back.
Even if you try to hide the fact that you slept with this girl, the truth could come out eventually. Nothing stays hidden forever.