I'm going to catfish my boyfriend, is that fine with you?
Oh? That is surprising. Why do you want to catfish your boyfriend?
I want to see if he'd cheat on me.
It's rude to try to catfish your boyfriend in order to see if he'd cheat on you. Partners are supposed to trust one another to be faithful. If you try to catfish your boyfriend and he finds out then he's probably going to be very upset and possible break up with you.
You're not just gonna stop me because I like the idea.
You need to just trust him unless he has shown you signs before that he is unworthy of your trust. Then you should get another boyfriend who is much nicer to you and won't ever cheat.