Polygamy deserves the same respect and consideration as same sex marriage.
Most people know the difference between marrying one, and marrying two at a time. And most people, straight or gay, have no interest.
Now, as to whether it should be legal, the christian anti-gay conservatives who support it should take their fight to the legal courts and the courts of public opinion and prove that it does no harm, and is actually good for society, just as interracial couples and gay couples have done.
As it stands now, it seems to be primarily a heterosexual religious conservative notion that only happens in very closed societies, and seems to be connected to the abuse of women, and even young girls.
If you disagree, get out there in public and prove your case.
Unless of course it's simple a straw man that you harp on because you don't have any other logical arguments.