I game should be built around pregnancy or not have it.
I'm not sure that I agree with that. I love to have pregnancy as an option / risk. Something that you can prevent, but also something that you risk if you aren't careful. I often find that games which has pregnancy as its main thing is actually kinda boring. Though, it should also play a role. There are too many games out there where pregnancy is in the game, but it is so shallow that it should've been left out. Many RPGM games are like that - a kinda "you are pregnant - 2 minutes later - you born a baby - now you can do whatever again."
On to the question of the OP: I don't really care if incest games have pregnancy. Pretty much all incest games has it as a complete given that your relative would like to bone you. I really, really dislike that. Incest is taboo, and your relative shouldn't readily jump you just because they are horny. Yes, this is an adult game, but I play them because they are captivating to me than just watching a video on Pornhub.
I'd much rather have it that your relatives will have some serious doubts if they would actually want to do it, and perhaps even turn you down. Obviously, that won't really fit into a game situated around incest.
So, to wrap it up. Pregnancy is often nice in a game as it can work as a punish/reward/whatever of your actions. In an incest game, I'd much rather have it focus on some moral dilemmas and not work as if you're just boning a young gold digger etc.
