Genesis 2:24 - Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Notice wife is singular.
1 Corinthians 7:2 - Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
Matthew 19:5 - And said, For this cause shall a man leave father and mother, and shall cleave to his wife: and they twain shall be one flesh?
Matthew 19:6 - Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder.
Mark 10:8 - And they twain shall be one flesh: so then they are no more twain, but one flesh.
1 Corinthians 6:16 - What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh.
Ephesians 5:31 - For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.
It's true that some of the Biblical patriarchs had multiple wives. But in almost every case the bible points out the problems that occurred as a result. Both the long history of Arab/Israeli wars as well as the destruction of Solomon's temple can be linked back to the problem of multiple wives.
I'll skip belaboring the point here. Reading of that text could be interpeted that way, but others allowed for multiple wives throughout the Bible.
However, just because it is believed immoral by some religions does not give you a right to codify it into law.
And for the record : Anyone who wants two wives needs his head examined.