That's a stupid idea.
There is no slippery slope. You must be a competent adult human being to get married. That's the baseline... after that, it's nothing to slope on.
That exempts children, animals... all of that shit.
There was a time when conventional wisdom said that society's baseline was one man, one woman.
Slippery slope, no doubt about it.
What is stupid and naive is to think that all the weirdos will be satisfied that polesmokers and carpet lickers can now get "married"