So, if the one man one woman marriage is God's form of marriage (only) then I suppose we must believe that polygamy is wrong and that such "additional" marriages are invalid.

Now Abraham only had one wife at a time (marrying again after Sarah dies) but he did have multiple concubines who gave him children (Gen 26:1-6). Why is he never condemned for it?

Jacob married Leah and Rachel and also had children with his concubines. If only first marriages are valid, then presumably Rachel and the concubines children are illegitimate, making the tribes of Naphtali, Dan, Gad, Asher, and Joseph also illegitimate? Again, not a word of condemnation.

David, Solomon, and most of the kings of Israel also had large harems, yet nothing is said against that either.

Why, if polygamy is so wrong, is God so silent when almost all of the great men of the OT practised it????

As for the Matthew passage, that states a man and wife become one, but where does it say that is the only possibility? Indeed Paul seems to indicate that such a union is formed any time two people have sex (1 Cor 6:16).

I know it is the "standard" Christian interpretation, but I see far too many Christians simply assuming that the norms of their society are Scriptural without actually knowing why (or even if).

As for the infection argument that Ake uses above, surely that is a complete red herring. The statistics there show that a promiscuous culture is dangerous, but how is that relevant if we are talking about three people who want to form a stable, exclusive relationship? Surely we should be encouraging just that if we want to slow down the spread of these diseases!