I'm sorry, are you sure you're speaking about impregnation? Not about some deviant or perverted thing but the most normal thing there's in sex. It's fetish (or kink), just like incest, but, like incest again, it's what society made wrong, not the thing itself (yeah, there's nothing wrong with incest, it's only societal taboo, nothing more). Impregnation never violates any societal norms, even in fucked in head western countries' societies impregnation does not violate societal norms, quite the opposite. That's the reason the marriage is for, the reason the sex is for (seriosly, that's why we having sex, only for that). And you cannot have new family members without impregnation. It's really the most normal thing 'cause it's the only natural reason for sex, for making love.
Why is that a kink, loved by some (myself included)? Besides the simple instinct, that is engraved in human males' behavior, I think the main reason it becomes a fetish - impregnation means pregnancy, pregnancy means children. And the sad fact is not every woman wants to have children with you (even your wife not obliged to want it). And maybe you both want children but you are sentient enough to realize that your finances doesn't allow you that. Whatever the reason you don't have what you want. And if some game give us what we want (if there's in-game "you", I mean male MC we playing), even in this virtual form, it's awesome.