Now that you've put that into words, it kinda makes sense. It's because the conservatives don't see queer relationships as equally valid or serious, even if the end goal is the same. That plays a lot into identity: they identify themselves with their "traditions", which includes "family", so when someone different tries to do the same, "oh, they're destroying 'The Family' by trying to have a family!".

It's kinda like gamer guys™ when a gamer girl just exists: "oh, no, she's a gamer girl, let's pick on her! You can't like the same things as me because of your gender!" In the end, conservatism is all about identitarianism ("you can't do x, y and z thint if you're a Man!!")—even though they'll blame that on the Left, because every accusation from the Right is a confession.