Because it doesn't matter if he's married to her. A wife can choose to abort her child, and the husband, under US law, has absolutely no say in the matter.
A wife can put their baby up for adoption, with the husband only getting the baby back after a long legal battle, and with no consequences for the wife.
A wife can decide she doesn't want to take care of the kids, file for divorce, and give the husband a choice of taking custody, or her asking for financially-crippling child support payments. And if she uses those child-support payments for partying instead of the kids, he has no say.
The fundamental incongruity of modern society is fathers having legal responsibilities without legal rights.