Personally, I think it's up to you and your partner to decide if it's alright for you to opt out and just watch if you're not attracted. If it doesn't bother him to play while you watch, and you're comfortable with that, then go for it.
As long as you have good communication between you and he knows why you don't want to participate, I don't see a problem.
DragonsLair
He is T. I am A.
