For the entire 1/2 million years preceding us, the word in each language for "marriage" has always meant (1) the male/female couple (2) in the context of a village's way of life (gender roles and rules).

For the entire 500,000 years that humans have lived in villages (groups characterized by a "way of life" defined by roles), the (sexual) coupling of a male with a female has been understood to be a vitally important part of the "way of life" of the village. Over the entire sequence of 25,000 successive families that succeeded at the game of life in order to produce you, people understood from both experience and tradition that copulation results in a powerful and enduring bond between the male and female. Such couples had an evolutionary advantage in producing offspring who would themselves be successful as parents. If, instead, our ancestors evolved from lone hunters directly to mixed sex groups with undifferentiated roles and then to villages with gender and other roles, the development of gender specialization could not have proceeded without such bonds and instincts. As groups evolved to exhibit gender and other role specialization, those groups that developed such emotional bonds and instincts would have an advantage in the competition with other groups (and with other species) for scarce game and water. As such "ways of life" developed, they came to include symbols and rituals which expressed the gender roles and rules applicable to the male/female (sexual) couple, and thus the concept of "marriage" emerged.

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 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.