Because it feeds (hehe) into their narrative that they are in some way desirable. If porn exists of it, it must be okay, right? Well, no. They forget that porn actors are PAID to be there. A person can feign interest in just about anything for the right price. If there's porn of a fit man and a fat female, they think that's realistic, but they never remember that dude got paid to act like he was into the she whale. But it gives them hope.