Marriage is about God's bond between two people; just because it's institutionalised, doesn't degrade what the whole point of marriage is, you're still going to church and you're still performing god's vows.
That‘s what I‘ve tried to bring across: in the bond between two humans god is only secondary; it ain‘t "gods" bond but the business between the two involved.
If you are religious that‘s where you‘d obviously disagree since everything is sort of god‘s work in your eyes, but imo you simply cannot force that belief on other people, especially when it comes to literally excluding so many from the institution of marriage (which is what I meant with 'gatekeeping'). Also, at least in Germany my experience is, that more and more simply go to the register office to get married and have a big celebration afterwards.
I get your point though, but I still think that even when it comes to the church refusing to marry gay couples is very primitive and this intolerance is one big reason why more and more are currently leaving the church.