That's a rather odd declaration, because claiming it is the sin of adultery leads to the conclusion that we all may expect to see the Patriarchs burning in the pits of Hell, for almost all of them had plural wives.
I will agree with you that MOST forms of POLYGAMY are indeed sin, but POLYGYNY is not a sin. That is proven within the scriptures where it was revealed by Nathan the prophet that God gave to king David at least two of his already plural wives, and the Lord also COMMANDED the taking of an additional wife (in relation to those brothers who were already married) from his dead brother who had no heir.
So, in the end, were it sin for a man to have plural wives, we would not see the very actions and commands from God that we can read by simply allowing the word of God to speak for itself rather than to force meaning into the scriptures that ultimately condemn the very Patriarchs of our faith. Talking to some "priest" isn't going to change what the word of God says, but those men will teach you the traditions of their respective institutions that may or may not align with the very word of God.
Now, MOST men can't handle more than one wife, and many can't even handle one, so POLYGYNY is not for all or most men. Most should avoid it. The men I know personally who have more than one are men with great love for others. They are all exceptional in their own way, as are their wives. With feminism having infiltrated so many cultures across this earth, along with the utter ruin it brings into the hearts and minds of its adherents, it was inevitable that socially engineered theologies would spring up in various sects, denominations, non-denominations, religions, government, laws, etc., with religious leaders chiming in with their false teachings about it.
Folks, REAL marriage, as defined by God rather than mankind and his petty laws, as defined in Genesis 2, is NOT that silly piece of paper from your local government, and the supporting laws behind that piece of peper. TRUE marriage is a life-long commitment toward inner change and growth. It is a sacrifice. It is a dying to self. It also is known to have a prolonging effect upon those who are truly committed. Those who see marriage as defined by that piece of paper from City Hall and laws, they are those who go running to get a lawyer when things get tough. Believe it or not, there are MANY couples out there who have one of those pieces of paper from their local government, and yet who are living in adultery and fornication after having been married, illegitimately divorced by governmental decree, and re-attached to someone who, in God's eyes, is not their spouse for the taking.
So, the question remains....what do YOU choose to believe; the Bible, some "priest" or "pastor," one of the man-made, socially engineered theologies that literally fill pulpit teachings, Sunday school materials, "christian" books, radio, TV, and in discussions with family and friends? Taking responsibility for what one chooses to believe is utmost in the exercise of TRUE faith.
Jr