Indeed, but if the reasons given are based on its likely future consequences, then they can be evaluated against the actual experience of places which are already there.
Consequences can take decades to materialize, and past history suggests they often do. But noone has that much patience, so
Sure, but what are the actual bad consequences of gays getting married, assuming one is not religious?
Monogamy is good and provides stability and emotional support even if it’s gay. Marriage generally comes with not being promiscuous, and sexual orientation is, for most people, not malleable, so it’s not like it will increase the number of gays - and the number of gay people is relatively small.
The only thing I can think of that’s a problem is that social progressives won’t be able to use it to signal how #woke they are anymore and they’ll move on to some bullshit that’s actually harmful, like legalizing polygamy (for normies).