For the entire 500,000 years preceding us, as the concept of "marriage" evolved, its purpose and function within the "way of life" of a village was always to symbolize and express the gender roles and rules applicable to each coupling of a male with a female in the village.

For the reasons just discussed, each such coupling was distinct from all other kinds of relationships, and such couples were the foundation of the entire "way of life" of the village.

Oo males out hunting in the jungle to provide for their village might have decided to "butt fuck" each other, might have even done so regularly, and might have even developed affection and caring for each other. But such a relationship, even if characterized by strong emotional and instinctive bonding, would not be a "marriage", for the simple reason that such behavior was not the behavior that was the subject of the "marriage" and gender roles part of the village's "way of life". Such behavior would not have been likely to have been consistent with the "way of life" of any village, and so would not have been codified in its "way of life" at all.

But even if such behavior benefitted the village and was sanctioned by the "way of life", it would not have come within the village's concept of marriage. It would have some other name, for the simple reason that conflating the oo behaviors would serve no purpose and would simply create confusion.