Here's the way I see it...
In the 1950's (I'll take that as an example) gender roles were clearly defined. Dad went out and made a living, Mom stayed home and cooked and cleaned and minded the children. But suppose Mom wanted to be a doctor or lawyer, and had the skills for it. Tough luck, she was a woman and therefore had no place in the business world. No penis, no paycheck.
Nowadays, if Mom wants to be a professional, she can do it. She's not excluded from the workplace simply because she's female. And by the same token, if Dad wants to stay home with the kids, he can go right ahead. This is a recent phenomenon, and still predominantly an American attitude toward family structure.
Both women and men have a lot more freedom now in the path they take in life. A woman who chooses to be a SAHM isn't "wrong", she's just made a different decision from a woman who's chosen to have a full-time job.
But some men are threatened by the idea that their wife would want to work outside the home and be successful in an arena traditionally reserved for men. The reverse isn't true - women are generally glad for the help their husbands give them with cooking, cleaning, and looking after the kids - and a little kid can be quite a handful even for two people
But there are a lot of men out there who want the typical '50s family, where there is no question but that Mom will stay home with the kids and he will be the unquestioned master of the house - and it's hard to find an American woman who will agree to such a situation. I certainly don't agree, and I wouldn't marry a man with that attitude, but there they are.
[stepping off soapbox]
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