"Marriage of more than 1 wife"
2 Samuel 12:8
I gave your master's house to you, and your master's wives into your arms. I gave you the house of Israel and Judah. And if all this had been too little, I would have given you even more.
This is old testament but it is a word of God which cannot be changed, not a word of another man. so this isn't a matter of old or new testmaent.
Abraham sold his wife 2 times for sex, for wealth. but this isn't a sin because it was on his permission. remind you of someone? a PIMP?
-In christianity sex with another mans wife is ok, only if you ask. but if you dont ask its a sin.
Resource: Christian site, so dont think i got this from an Islamic site
[Ethreal i need you to respond to this not EMPIRE if im wrong PLEASE correct me] thanks bless you all