Simple: because the public would not accept it.

He felt the time was ripe to take a different stance on Gay marriage, finally allowing something that the rest of the Western world has  allowed for ages. (not that long actually, but long before the States). The fact that he even dared come out for such a stance, which still holds a lot of controversy in a religious country like the USA, with some regions being borderline extremist, already shows he has some balls.

A Republican presidential nominee would never be able to make these kinds of claims, even if he was for same sex marriage, because his own party would slaughter him.

And that brings us to the problem of a limited-party democracy, especially if said democracy is essentially and practically limited to only two partie, where either of two parties needs to get a majoritys: politicians will be afraid to take "extreme" stances, in fear of estranging a large part of the voting population in the states, which could just be that handful of voters that swings the votes into the favor of your rival, and loses you the election.

Something like these "loving unions" as you call them, won't be accepted for a long time. Heck, even in the more progressive European countries this would still be a very weird stance that would estrange one from a lot of possible voters. Perhaps in a state with a majority of Mormons, such as Utah, would this law have any chance of passing. No where else though, especially not in the States.

So after all, it is all just politics. But I think you yourself also clearly knew this answer, and this question was rethorical. On that note, why am I even replying? I don't know.

The federal government shouldn't be involved in marriage.

There's no reason those suffering from psychological disorders (homosexuality) should be getting tax benefits which I don't get because I choose to have 3 wives (can't legally marry them).

It's immoral because it judges and discriminates (in favor of those who cannot, by form, procreate and create families, no less), and it's unconstitutional in the USA under the Fourteenth Amendment.

im definitely against the anti incest laws.  if i dont plan on having on mutant kids i should be able to have a threesome with my extremely hot twin cousins

mutant inbred lawyer cousins is on a level with zombie invasion and that's just because viral zombie runners are pretty bad

I thought it was "You met President Grant?"

it seems like it since the only test for homosexuality is self-declaration.

I'm pretty sure there's another test...

it involves sexual stimulation

you can do tests that examine involuntary sexual stimulation though

unless they did it to some sleeping people...

1 down 3 to go!

""The pupil reacts very quickly, and it is unconscious, so it's a method that gives us a subconscious indicator"

That's not true though; as a four-eyed freak I know well that my focus is consciously shifting to what I THINK should be seen more clearly.  I saw it, but my eye left it out of focus, my brain interprets it as something that should be stared at, and THEN my eye moves to it.