Marriage is social convention, but it rests on biological realities.
Over the course of four million years, biological evolution has selected two sexes for the reproduction and rearing of offspring to perpetuate the human species. Human reproductive organs possess an evolved compatibility. The male's external reproductive organs and the female's mostly internal reproductive organs when properly functioning enable intercourse and reproduction. Although instinct does not determine human behavior the same way that it does in other species, natural processes involving various visual, auditory, and olfactory cues influence human mating. Facial appearance, voice pitch, and the more subtle influences of pheromones such androstadienone and copulance draw the sexes together. Sexual desire itself as a visceral urge is expressed through human reproductive faculties. These biological phenomena assist in establishing human mating generally and finding genetically advantageous mates specifically.
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Men and women have been engaging in sexual intercourse, giving birth, rearing children, and forming extensive kinship networks based upon consanguinity for ions of time. And over the course of time, different societies have developed a variety of social norms or customs to regulate and infuse meaning into the forming of families. Those customs are what makes marriage a social convention. But marriage is a convention that rests upon marriage conceived as a union between a man and a woman.