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As a happily, monogamously, heterosexually married man, let me suggest that your standard definition of marriage is pretty narrow. For a broader definition of marriage (less attached to a specific context and applicable to humans throughout the world and across time), consider the following anthropological definition:
"Marriage: A culturally sanctioned union between two or more people that establishes certain rights and obligations between the people, between them and their children, and between them and their in-laws. Such marriage rights and obligations most often include, but are not limited to, sex, labor, property, child-rearing, exchange, and status"*
*Haviland et al.
2008 Anthropology: The Human Challenge, 12th Edition.