Ok. A woman is married to man. She decides that she would like to find a new boyfriend.
The boy friend and her actually have sex. The relationship with the husband continues and the relationship with the boyfriend comes to an end.
Now there is an entire book in the bible that describes this type of relationship. The relationship where the wife left her "first love." Hosea.
In a general sense, this type of thing is "nicely" called polyamoury as it is a woman who loves more than one man. The same can go for men. One man can love more than one women.
In some sense. People say, oh this type of thing happened in the bible times.
Yes. It did. The bible describes it.
The clue here about why polyamoury isn't biblical is the clue in bearing of children. Ok.
Woman has multiple lovers. Which one is the father? It is not biblical response to life to not know whom the father of a child happens to be.
Furthermore, some polyamoury supports and even encourages LGBT behaviours. Ok. So it is ok for two sister wives to do to each other what they do with their shared husband.
It appears to be loving, but really it is deception revealed by the forces of nature in "the flesh."
Simply ask a few followers of polyamoury how they feel about biblical things. Negative response. Oh, the bible encourages men to oppress women. Lie Lie Lie.
Ok. State that it is best for a woman who is a "secondary partner" in a relationship to wait until she is 25 years old. Ok. The response to this is.......oh you are so judgmental. Reality is that a woman is not developed in her mind and heart until age 25. So the tampering with nature is not so great in the long run.
All in all. You will find bitter, hatefilled remarks from "polyamoury" circles, because they do not submit or awknowledge biblical truths.
10 Blessed are they which are persecuted for righteousness' sake: for theirs is the kingdom of heaven.
Now I grant, there are certain instances where polyamoury in a BIBLICAL tone is practiced and encouraged in the bible. Those things never encourage breaking of God's laws however. Strictly platonic outside of marriage boundary lines.
Now, I do not hate people who want to experiment and try "swinging" or whatever. Infact, they may be less likely to be hypocrite if they are true about what they do. But justify their own actions as more righteous than the actions of Christ on the cross, well, that just isn't a case to support their lack of sense.