I'm going to remain best friends with a serial cheater
Why do you want to remain friends with someone who constantly cheats?
I have couple of reasons for this, first, I am the only friend he has, second I really love him.
It is nice to be friends with people, but you should tell your friend to not be a cheater. That is bad behavior.
I don't think I need to do that.
I think you should do it. How would you feel if your girlfriend was cheating on you with your friend. It is not right to treat others that way.