One of the incidents of marriage is that it creates a legal presumption of paternity. That presumption operates independently of the genetic facts. We have no idea how many children of opposite-sex married couples are in fact not the biological children of both parents, but I’m going to hazard a guess that the absolute number who are not is likely to be as high or higher than the absolute number of children of same-sex married couples.
It’s discriminatory to allow that presumption to operate for an opposite-sex married couple, but not for an opposite-sex same couple. This would unfairly privilege the children of opposite-sex couples who are not, in fact, the biological offspring of both parents, as compared with the children of same-sex couples.