Holy matrimony is between one man and one woman in the sight of God.
Your interpretation of 'Holy matrimony' - which you are not alone in cleaving to - is one man and one woman in the sight of God. Bearing in mind that, even within the tradition of that particular deity that's historically not always been the case - Solomon seems to be a pertinent example with his seven hundred wives and three hundred concubines... That notwithstanding, how do you reconcile the 'one man, one woman' party line with intersex individuals? Are they prohibited from marriage?
It is just one model of matrimony among a whole host of others none of which has a monopoly unless the powers of law deign it to be so.
And the sexuality-neutral model doesn't claim a monopoly - it lets you choose whichever interpretation of marriage you want for you, but it doesn't let you impose your model on others.
That formula as it happens served to distinguish Christian marriage from polygamy.
Except that there are Christian sects that practice polygamy, with a number of parallel 'one man, one woman' marriages, so it does nothing of the sort, notwithstanding that polygamy isn't the issue under discussion.
Gender neutrality was not a thing then or for a long time after.
And, as has been explained, this isn't about gender neutrality, this is about sexuality-neutrality.
O.