I have studied this topic much more than I expect you have, though I suppose I could be wrong. One man and one woman = one flesh. YES. And one man with another woman = one flesh. YES. See 1 Corinthians 6:16.
Regarding “own” wives and husbands, I encourage you to do a review of the original language and then tell me if your stance is the same. See: http://biblicalpolygyny.com/wiki/index.php?title=Own. Why would Paul use a different word? Why would he use a word that means what “own” means when referring to the wife?
Well, in the last 33 years I have read the Bible perhaps a 100 times or more, studied it diligently, read entire sets of biblcal commentaries (OT & NT), have a BA in Biblical Studies, an MA in Christian Counseling, and have spent years on my face before God seeking after His will, purpose, and plan. You may also wish to entertain some thoughts on Paul’s advice to Timothy & Titus regarding qualifications for the man of God being that of the “husband of ONE WIFE,” (1 Timothy 3:2; Titus 1:6). Combined with the “whole counsel of God,” this qualification would apply to all who “name the name of Christ.” God’s plan for man was one husband, and one wife.