Human beings certainly have a greater variation of behavior than other kinds of organisms but that should not obscure the existence of norms. The norm for human beings is monogamy: a marriage of one man and one woman. The existence of variations from that norm and failures to adhere to the norm do not invalidate the norm. Monogamy is rooted in biological, social, historical, legal, religious, and moral realities. There is nothing unfair or unjust about monogamy. It is fully justified, rational, and moral.
‘Sexual’ is a biological term that refers to the way a species reproduces. Sexual reproduction means reproduction by two members of a species who are of different types (called sexes), one male and one female. Other animal species also reproduce sexually. Some plants are asexual, meaning each member has the means to reproduce alone.
The term ‘heterosexual’ is a redundancy. ‘Hetero’ means ‘other’ or ‘different’ but that is just what sexual reproduction means so ‘heterosexual’ means the same as ‘sexual’. The term ‘homosexual’ is a contradiction in terms. ‘Homo’ means ‘same’ so ‘homosexual’ would mean reproduction by two members of the same sex, which is not sexual reproduction. So it is self-contradictory or means something like ‘anti-sexual’.
Polygamy is a primitive form of marriage that allows more than one man and/or more than one woman in marriage. So-called same-sex marriage is thus a form of polygamy. This is not progress–it is a turn toward primitive ways. Since marriage is foundational for societies, this indicates that modern societies are betting the farm on sexual tomfoolery.