In the eyes of God, a "husband" or "wife"... is one with whom one has been "joined" by means of flesh. That means... sex. By means of the sexual act, where two fleshes join, the couple become "husband and wife"... ONE flesh.
Man behaves as man behaves and something that they do is marry - this behaviour has been used as part of a teaching to convey understanding - just as other animals behave in other ways, and the observation of those creatures are used for teaching Does not nature teach you?
I took them by the hand to bring them out of the land of Egypt, my covenant which they broke, though I was their husband, says the LORD.
For your Maker is your husband, the LORD of hosts is his name.
The application of this spiritual teaching to the flesh has been very burdensome to many people (eg. the battered wife), but it is one that is still often necessary to live in some harmony among a great many people.
For it has seemed good to the Holy Spirit and to us to lay upon you no greater burden than these necessary things:............and from unchastity.
Since in the eyes of my Lord, relations with another while having a living spouse constitutes "adultery"
It's simply what adultery is in the eyes of people - some people.