It is very easy to become caught in the linguistic and logical thicket such thinking is based on, but in the long run it is of the form, “A procreative act is an act which, if the couple were capable of procreation, would be capable of procreation.” This strange thesis entirely begs the question, in part because “acts” aren’t procreative — people are. Procreation is not a matter of form, but of substance and capacity. If the couple aren’t capable of procreation, no act they perform can be “procreative.” In the long run this is just another way of framing the assertion that only men and women can marry.
To get a bit technical, this is the assertion of a counterfactual conditional as if it were an indicative conditional. (The difference is between “X would be if Y were though it isn’t” and “X is if Y is and it is.”) Saying something “is” when it “isn’t” — though “it would be if it were” — is at the heart of this logical fallacy.
One of the exponents of this view declared that it is analogous to angling: even when a fisher fails to catch fish he is still fishing. This is a misleading analogy. The proper analogy, from the traditionalist perspective, is “sex is to procreation as angling is to catching fish” (i.e., sex and angling being the activity and procreation and catching fish the particular and assumed desired result of that activity). Sex without the possibility of procreation is sex, but it is not procreative. It is nonprocreative sex — whether whatever makes it incapable of leading to procreation is artificial (birth control) or natural (temporary or permanent infertility, or the sex of the members of the couple).
The traditionalists simply want to restrict sex to those who formally embody the capacity for procreation, whether they are actually capable of procreation or not. And that is just another way of limiting sex, and marriage, to a man and a woman: the very premise that needs to be proven. And so the wheel spins another round.
Tobias Stanislas Haller BSG