Perhaps an assumption that one is in it for the long haul. If your parents were married till one or the other's death, or at least past the date of your own wedding, then that will affect your view.

Kids probably don't hurt. Samuel Johnson said that monogamy was so unnatural that it was all that the law could do to keep couples together. (And he had been happily married.) But married parents do offer stability to children--not always, but probably more often than other household configurations.