There's a great argument for you. You could say the same thing about wife-beating, slavery, and blood feuds.
Jacob, David and various other Biblical figures had more than one wife at the same time and some had many concubines as well.
If Biblical precedent doesn't work for you than the code of moral law you abide by is yours alone so why subject everyone else to it.
In America a man can impregnate 5 willing women and if he is rich enough, be forced to provide financially for each of his children, as he should. If those 5 women all decided they wanted to live in the same house with the man and all care for the children of the same father, the law is against it.
It seems to me homosexuals want the government to recognize gay marriage to not only be accepted as normal behavior but also to reap government benefits like tax breaks for the married.