I love this fantasy, and I'm glad I'm not the only one so it seems. When I am being fucked by a top man, I fantasize about not getting spermed but if his sperm is good enough for me. I look at a man and wonder what if our genes were mixed, would we have beautiful mixed race kids, and who will look like whom. If it's male, will our kid be hairless like me or hairy and bushy like his dad's crotch right now which is thrusting in and out of my wet, silky butthole about to bust a DNA nut?
I also think of it the opposite way when a so-so top fucks me. My body responds to the sex, but the back of my mind will want him to pull out when he comes, because of perceived inferior attributes. If he has bad teeth, bad skin, bad hair - I don't want that gene passed on. If he looks poor (LOL, I know superfluous) and can't look after himself. Not that rich looking tops get desired either (if they have bad teeth, bad hair, skin, etc).
Our brain is our biggest sexual organ and if we use it wisely, sex can be fun.