if you want to say no, say no. playing something you're not comfortable with ends badly. if you want to find a nicer way to word it, try something like 'i appreciate the offer, but i think we split a little on where we think the headcanon for this particular relationship goes and i'm not very into anything abusive with these two since i don't feel there any canon to back it up. maybe you can find another version of x character to play that off of.'