People like to justify an act simply because it was carried out by men of God in the Bible, but does that it make it right? For example, was it right for David to have so many wives?
The Bible says, in reference to a king, "he shall not multiply wives for himself..."Deuteronomy 17:17
So there you have it! It wasn't right.
Now, if it isn't right for a king to multiply wives for himself it isn't going to be right for anyone else.
Also, concerning Adam and Eve, they became one flesh. And that is the ideal of marriage, that the couple become one flesh. In other words, they share an intimacy between each other that is more than a typical friendship.
And really, how can a man become one flesh with a woman when he has so many other wives to become one flesh with? Do you see my point?
He can't become one flesh with any one woman because he is sharing himself with so many women!
These guys that are telling you that it is Biblical are wrong. And to be quite frank with you, I wouldn't trust them to teach you what the Bible says about anything, since they are using it as a tool to suffice the needs of the carnal nature!
Allow those who are well-balanced in the truth to teach you. But you should study for yourself. Even somone who may appear to be holy on the surface could be, as Jesus would say, a wolf dressed in sheeps clothing.
It is always best to study for yourself. Allow others to teach you, yes, but make sure you take everything that they tell you to the word of God, and that you do that prayerfully, to see if what they are saying is actually something that is according to the will of God.