In Western society, the act of marriage confers (often vastly) increased wealth and status to the lower status partner, and it is usually women who prefer to "marry up". Furthermore, it is also vastly more fulfilling and satisfying to care for one's own children than one's mistress's children. No two roles could be more different, frankly.
Speak for yourself. I'd much rather take care of other people's children. I can go home to a nice, child-free house, as I have no desire for children of my own.
it is usually women who prefer to "marry up"