Because they don't work.
Say two adopted children, let's push the enveloppe, of different race. Shouldn't they be allow to marry? What would be the rationale?
Too close for comfort? How about marrying your childhood sweetheart. Awww isn't that cute. They've been best friends for ever, now they're together. Well...
Would you marry a woman with a couple of side-hubbies? Maybe you're that kind of guy.
Plenty. Oh but you mean GOOD arguments? Well, who knows, if I only I could hear them above the noise. If you want good arguments, you need data, not 'feelings'.
On the other side, apparently, homosexuality is a choice, or at best a disease. Couldn't be anything else, you know, God wouldn't be so cruel. So there's that.
And it's icky. Well, it is, but I can think of a few more important things that would make me turn my back against a wall.
BTW, I lived the best part of my life surrounded by really interesting neighbours, most relevant here, two sisters living together. I don't want to speculate, but here they were sharing an apartment, always together, never married.
They've been together for as long as anyone remembers,They were the sweetest old ladies you could ever meet. Catholics too. So you see, they aren't all bad.