I can't really put into words why I find it all so appealing, but it's a very basic and hard coded part of my sexual self. It's been something that's gotten me excited basically from the point that I went through puberty. I remember being in high school and having pregnant girls in my class, and it was just an incredibly arousing thing to even be around them. From very early on, the very thought of the girls getting fucked and knocked up just pushed all of my buttons really hard.
Back them, porn didn't deal with creampies much and it was mostly cum shots on the face or tits. But I got into erotic comics and manga/hentai, and was incredibly turned on by ones where they would show the cock slowly pulling out of the woman and huge amounts of cum leaking out of her pussy, and hanging off the guy's cock. Stories where the guy would knock up the women, especially if they were his family, coworkers, or basically anyone not his wife would just blow my mind.
And from there on, its always just been a huge fetish/fantasy thing for me. Of course I know in the back of my mind its not real or how the world works for people to operate that way, but that's not what porn and erotic entertainment are about. Its fantasy. And for me, a game where the MC can build a harem and have the girls absolutely begging to be knocked up is just as hot as it gets. There's also a subset of real porn where the girl is just begging the dude to cum inside her, and sometimes knock her up and I love it. Can't get enough.
I also, from the very beginning, have been most attracted to girls with very "womanly" figures. Big tits, very wide hips, thick thighs, soft bellies. The kind that just look fertile, with "child bearing hips" as they say. And then the whole lactation, breastfeeding fetish from that point as well. I love every aspect.