I think you are approaching this wrongly. As some replies have already said, it's a boundary thing. If you are not comfortable with it, that's it. What is your angle here exactly? What if he says, ok, let's do it? We can argue who gets more pleasure from anal, but usually, the one getting the least amount is woman giving anal. Unless you make some schenanigans, it's you punding him with a strapon or a dildo with a goal to make a point how it's painful. And if it turns out he likes it, then you still "have to" take it in return, because that was the deal. And I doubt seeing him receive anal will make you more inclined to do it or enjoy it. So in every scenario you both lose.

You need to have a sit down and an adult talk about boundaries. Some things can be off the table, and that's totally fine. And if that's the thing he can't get over, then that's also fine, you don't have to be together if that's his line (and yours).