This one always gets wheeled out.
Mothers give birth. Fathers are very important (I have a fabulous one) - but mothers are basically better at looking after children than men are, because they are hard wired to do so.
The whole 'equal parenting' thing has only really come into being since women had to work because a mortgage on a decent house is basically impossible on one salary now. How many mothers genuinely want to work ft?
Obviously there are some crap mothers around, but on balance, women are better at it than men are. My dad is, as I said, fabulous - but he wouldn't have been fabulous if he'd been a SAHP. He'd have hated it. He played an entirely different (equally important, but different) role in my life, as he still does now.
I could either have worked (and did, pre-children - had a super-duper high-flying professional job) or could have been a SAHM because I had a private income. I chose to be a SAHM, because I wanted to, and because it was better for the children.
Someone upthread said they would be bored as a SAHM. I was never bored. I did very little housework (because that really is boring) and did basic cooking but am really a crap cook (for me, cooking is also filed under 'boring'). When the DC were little, I took them out and about, talked to them, read to them, played with them, etc, etc, etc, etc, etc. When they went to school at 5, I did voluntary work, went riding, walked the dogs, worked on my garden and vegetable garden, saw friends etc. The school days are quite short, and the holidays are very, very long. I also did a fair bit of looking after my friends' children while they were at work in the holidays, and a lot of ferrying children (mine and other people's) around. I was glad to do this - these were small, close-knit prep schools and I was good friends with all the other mothers and knew their children well.
There was no boredom. If I'd kept my job, I am sure there would have been some interesting elements to it, but I would only have coped with the boring bits and working with idiots if I'd absolutely had to do so. And I didn't, so I didn't.