I'm beyond attracted to my boyfriend's best friend
Have you thought about cheating on your boyfriend with his friend.
Well, yea the idea has been in my mind before.
While it's fine to find your boyfriend's friend attractive, it would be completely inappropriate for you to act on that attraction. Cheating on someone is very hurtful, and you shouldn't do it.
Nah, I'm going to end up with his best friend. jk
It would likely cause a lot of tension if you cheated on your boyfriend with his friend, so I really don't think you should say that, even as a joke. There's nothing wrong with liking the friend, but cheating would be wrong.