What if I was basically ruining my parents' marriage?
That's cruel. How do you feel about ruining their marriage?
I don't care about their marriage because I've had it with them and I can't take it anymore. I'm just going to cut them both off.
You shouldn't try to ruin a marriage. If you disagree with it then you should move out and let them figure it out themselves.
I can't stay out of it completely. I'm having an affair with my mom's boyfriend.
Well it does seem like it is not a good relationship if your mom has a boyfriend, unless your dad is okay with that. But if they are willing to work on their marriage, you should stop trying to mess up their relationship. You also definitely need to stop having an affair with your mom's boyfriend.