Liberals are disappointed that the document, issued by Pope Francis, doesn’t reform the church’s teachings on homosexuality. It reminds everyone to be nice to gay people—“Every person, regardless of sexual orientation, ought to be respected in his or her dignity and treated with consideration”—but supports cardinals and bishops who reject same-sex marriage. “There are absolutely no grounds for considering homosexual unions to be in any way similar or even remotely analogous to God’s plan for marriage and family,” says the document, quoting these prelates. That’s because, “according to the order of creation, conjugal love between a man and a woman, and the transmission of life are ordered to each other.”

I’ve defended this argument to friends and colleagues who support same-marriage. I don’t agree with it, but it’s not irrational. Strict biological distinctions between groups of people are rare, but this is one of them: Same-sex couples can’t produce biological children together. A rational person can stipulate that no relationship between two people categorically incapable of producing children together—i.e., a same-sex couple—can be a marriage.

That position starts to crumble, however, when its advocates exempt heterosexuals from the same analysis. Many opposite-sex couples can’t produce children together, either. For them, “Amoris Laetitia” offers a more generous message. “Marriage is firstly an ‘intimate partnership of life and love’ which is a good for the spouses themselves,” the document declares. Therefore, “spouses to whom God has not granted children can have a conjugal life full of meaning, in both human and Christian terms.”