That is a ridiculous and illogical argument for your case. It is far more likely for the scenarios I outlined to occur if "marriage" was left to the individual as you suggested. I suggest you try to come up with a more realistic argument to support your position.
It's not me who "doesn't get it", it's you.
Like it or not, this is no longer " a Christian country" where common sense prevails at the ballot box. We've elected a bunch of narcissistic idiots and lawyers to public office who will do whatever is popular and gets them re-elected.
Here in California the ONLY thing that got the "marriage between a man and a woman" clause successfully added to the State Constitution was the minority community -- Blacks & Hispanics. The Mormon Church did some in promoting it and Mormons became the primary target for a gay backlash, but that was only because gays didn't want to get called "racists". It was minorities who were the real numbers here in passing it.
But the demographics suggest that this will change in just a few years as a generation dies off and a new generation takes its place, because younger voters don't really care who "marriage" is between.
You can't hold the line with populism anymore, because the masses are against you.
The only reasonable solution is to take "marriage" off the table for government. We all get the legal protection of something like "a domestic contract" between two consenting adults and "marriage" stays where it belongs -- in the hands of the clergy and the private sphere, which is the only place that really regards it sacred, anyway.