Observed fact:  Gays were not able to marry in the US for the duration of its existence, nor in the colonies before, until the last decade.  No harm came to the US from this standard (Which was pretty much the world standard).  So, if SCOTUS rules against this issue, the safety and existence of the United States is not in jeopardy.

In the last 15 years or so, several nations and several states have legalized gay unions and marriages.  There are no observable direct or indirect repercussions causing damage to the political existence of these jurisdictions.  So, if SCOTUS rules in favor, the safety and existence of the United States does not appear to be in jeopardy.

Every opposition to the matter I've heard comes down to either "Our religion doesn't like it," or "We've never done it that way."

Setting aside the religious question as not admissible in court, we come down to, "At the time the nation was founded, marriage was between one man and one woman."

And for the last two months, the Left (including a large number of gays) has been bleating that, "At the time the Constitution was written, 'arms' referred to muskets'" as an argument against any firearm designed after the 1870s.

So, logically, there are no arguments for, and the Left's own logic against.

Argument in favor of gay marriage fails for lack of support, and lack of logical consistency.

Unless, of course, the Left would like to compromise and concede that as time progresses, society and technology do, too.  Then, they must apply that argument fairly to groups they don't agree with.