Steve isn't a father figure to those kids he's more of a big brother to them. The way the fandom took the whole Steve is a single mom joke so far is insane bc he isn't a parent to the kids, they have their own. Idk why the teens (esp Steve) get parentified in this fandom as if this is like something good and cute. Plus literally he doesn't even act like a dad towards them. He's the annoyed older sibling who just drives them around and gives awful advice. Yeah he says they gave them the experience because he's literally an only child, he never had to take care of someone younger than him. But this gave him the option to see that this is a future he wants to have. It doesn't mean he wants to start now like you said he can go wild for some time but eventually that's his end goal and idk why the fandom hates it sm when they started the whole mom Steve joke.
It's not wrong for him to want his own kids. You can find family in your friends, but that doesn't mean he isn't allowed to want something for himself. He grew up as an only child, his parents don't seem to love each other that much, idk what his relationship with them is like but it makes sense for him to want to settle eventually because the kids made him realize that he can do better. He longs for someone who he can start a family with, and there is nothing wrong with that. It's also not like he wants a family bc it's expected of him, he wants a family because he genuinely loves the idea. He won't be the classic dad who isn't really involved with a stay at home partner. He wants to be present and be there in ways his parents weren't. Like idk what show you watched where you came to the conclusion that Steve only wants kids bc of tradition or conformity.
Hi, it's just a head canon idea that I had. And yeah while I can agree that he would want kids of his own and he's want to start a family, the idea that he is expected to have kids and start a family can also coexist.
he can know that it's expected of him, and not want kids BECAUSE of that, but also want kids for himself and his own reasons. I think that's what I left out.
As for him being Mom Steve? I agree, I personally feel like, a majority of the time he's kept around because he's useful. He can baby sit the kids while the adults do the "hard lifting" and I know he hates that. But I don't see it as he hates it because he's an annoyed older brother, he hates it because the other adults, such as Nancy, Jonathan, etc, they also treat him as if his only use is to be a baby sitter.
And I'm not saying that Steve necessarily views himself as their dad, he probably wouldn't ever feel comfortable if one of the kids called him dad, but he can acknowledge that he is the parent friend and he takes care of them in a way that they're all lacking.
Whether you like this idea or not, as it stands, each one of the kids in the party are missing one thing in their life: a father figure. And no Steve is not anywhere close to their dad, but he realizes because of the party, his dream to settle down as soon as possible, get married, start a family, it can wait for a little bit. He's allowed to just live and explore.