One of the going theories (if you can call it that) is that in general,
woman seem to like masculine men more when they're ready for having children,
and once they do have children, they go for softer, more caring men.
It seems plausible to me.
When you want to make a child, you look for good genes. Men that are strong, or seem strong would provide such genes.
Once the child is born, a strong macho man is not as interesting anymore compared to a caring, loving father. (not to say physically strong men arn't good fathers)