Lumpyoatmeal saidI see it as largely a gender difference that's pre-programmed into us by our genes.
Women are fascinated by children. On the train to work if a woman was sitting by the door with a child or baby all of the women who came on through that door would at minimal smile at the mother and child, often telling the mother how beautiful their baby was. Men pass by without even noticing it.
From the biological propagation of the species perspective it's obviously better for the mothers to be interested in their offspring. Back when we were cave men and with the species before that the men were out hunting for food, being the providers, bringing home the bacon, etc. We never needed to develop any strong orientation towards children since the women were taking care of them.
You also see it in how we define what is a successful person, at a basic level. For men it means having a career; again being the bread winner and bringing home the bacon. For women it's a two step process; step 1 being having a husband, step 2 having children. A woman's identity is about their family, their husband and kids. A man's identity is about his job and career. Here in the suburbs you see many suvs or minivans driven by mommies that have on the back window the stickers of white outlines of the family, the daddy, mommy, and the kids. Good luck trying to find a man's car with that.
I actually seem to know more and more women who want nothing to do with having children. So while that statement above may be quite the generalization, i still think it is mostly accurate and I agree. (Seriously, so many girls are just obsessed with babies.)
Also there is the fact that gay men, if they had children, would have children that weren't biologically theirs. That does make a difference, as it somehow is easier for me to envision making all the life sacrifices required for raising children if they are a direct descendent of my genes. I'm aware this is another sweeping generalization and plenty of parents who adopt make excellent parents. I also think a large amount of straight people have children on accident. At least at first. Hah!
All that being said, I would rather have the time, sleep, and money. Woohoo.