1. It's *wrong*, because God says it's wrong. End of. It's extraordinary how flexible and pragmatic God would have to be if He existed, to decree Laws that match the prejudices of His followers so well. Declaring a New Covenant was a master stroke, allowing pick'n'mix from the Old while, of course, the actual provisions of the New can be quietly ignored as the naïve, leftist claptrap they are. Marriage-is-for-reproduction comes up too, of course, but hardly worthy of serious attention given child-free het marriages on one side, and surrogacy, adoption and (coming soon to a future near here) actual, if technologically aided, conception for Lesbian couples.
2. More worrying is the slippery slope argument: if chaps can marry chaps, what's to prevent them marrying their horses, eh? The obvious and essentially irrefutable argument that horses, like children or chameleons, can't give informed consent doesn't seem to have any impact. The only conclusion I can come to is that people making this argument don't believe in equal partnership and consent in cis/het marriages either: if a man can take a woman as his chattel with no right of appeal from her, I guess there's nothing but a non-existent God between a man and the sheep he would wed. May they be very happy together.