My wife and I have 7 kids, aged 16 to almost 2 (plus three miscarriages entrusted to God's mercy).
The distinction between what a guy wants when he's first married, and what he grows to want after having his first kid, is a helpful distinction, yes.
(Setting context per
@Infantry23: we were Continuing Anglo-Catholic most of our adult lives, now Orthodox, and in both communions a common view is that artificial contraception should not be a default, but that priestly dispensations can be made in exceptional cases of grave medical need. We did not use contraception for most of our marriage, but due to a medical emergency during our last pregnancy, our priest has now blessed us to use non-abortificient contraception to protect my wife's life.)
I haven't met many single guys who talk about wanting a baby. I agree that that would feel unusual.
(I have met a few single guys who say that they hope to have a family someday -- who seem to have in view the total picture, rather than the typical maternal desire to hold a baby in particular -- and that's different, I think.)
As a Christian, I do assume that God has built those maternal instincts into most women; but, a woman who wants a baby may not be able to answer "Why?" any better than I could answer why I want to climb the next mountain or take on the next unnecessary project.
When we were first married, the prospect of children felt distant and theoretical. Until I found out in the second month of marriage that we'd gotten pregnant in the first. However you want to explain it, my happiness about it was just as big a surprise as the pregnancy was.
Once you've had an 18 month old toddle towards you as fast as he can with pure joy spread across his face while he cries "Dada!" and hug your leg, you get it. And as they get older, there is no joy like watching your kids make good decisions and choose to be a blessing to others. I'd trade anything in the world for that, every time, even if I had a million lifetimes.
BUT, I didn't know that when I was married at 21 years old. I didn't get it yet.
(Maybe that's a plug for the traditional teachings of the Church: you can't answer "what do you get out of it?" until you've had a kid, and some of us need to be told to remain open to children so that we get the chance to "get it")