Impregnation and monster girls.
My upvote here assumes MC actually fertilises his lovers rather than the TiTS/CoC-verse trope of MC being the host to parasite eggs in their digestive tract. To each their own but I don’t see anything erotic in having worms and I would prefer this was always distinguished from procreative actions.
Also some games (MM for one) have faux pregnancy which involves a character having a pregnant appearance but not actually being pregnant and this totally misses the point.
The opposite of this, pregnancy without visible change, is a more acceptable compromise.
For me, the fundamental fetish is in accomplishing the absolute original purpose of our potent reproductive urges. It is the Ur-fetish. It’s also something that for many, many, reasons can only be casually indulged in fiction.