I assume the same applies when one spouse or the other has an adulterous relationship
Presumably, our whole society Christians and not have replaced the holy union of one man for one woman, with all manner of non-holy relationships, thus the militant homosexual community is demanding their "unions" regardless how temporary or open, be given the word AND meaning of marriage.
Sadly, the members of the faith community have gone along with easy divorce, pre-marital sexual relationships (not a brief relationship prior to marriage) either numerous partners or moving in with one another and even having children without bothering to get married, doesn't act like the term "marriage" means anything at all so why bother to object when sodomites want their unions sanctioned.
There is a considerable difference between homosexuality and the sin of adultery. When heterosexual adultery is committed, the person sins against his spouse, and the adulterous union is unholy. But the act itself is natural.
Homosexual acts are never holy and never natural.
When a heterosexual act is committed, even in fornication or adultery, the act itself conforms to the way God created man and women. The act is complementary. But when a homosexual act is committed, the person is turning away from the glory of God's creation in the complementary nature of the sexes, and turning towards an image of himself.
I was once told by a fundamentalist "preacher" that all sins are equal. That cannot be true; it would be a sin against justice to count the theft of a paperclip the same as the murder of a child. Some sins are greater than others. And so it is with homosexuality. Adultery is a grave sin, but the homosexual act is an abomination.