Did you notice your use of the term 'mate'?Why did marriage start ?
well let's speculate---some animals have herds of females with one dominate male who fights off other males. Some birds mate for life. I think Wolves mate for life also. We humans have laws that are good for monogomus relationships. Some human beings are homosexual, and as thinking human beings with laws, the laws should accomodate these individuals with equal rights to marry and get the
same legal rights of heterosexual couples.
It refers to mating behavior. Look it up.
Marriage is the cross cultural constraint on evolutionary mating behavior.
Gay couples are a defective desolation of the very core goal of evolution.