If we are to use the biblical sense of marriage, the Old Testament has a few incidents where a man was married to multiple wives[1]. Then if we are to be traditional, can a marriage not be among multiple people?
Since we have denied same-sex marriage on religious or moral grounds, are we saying we should deny the right for a man to marry multiple men or women or a woman to marry multiple men or women because of our beliefs? Isn't that the same reason that we're denying same-sex marriage(s)?
Age verification is OK, since minors aren't recognized to be capable of forming contracts.
The argument for denying plural marriage is that, in the US, polygamy has been primarily a tool for subjugating women. Legally recognizing these unions would be granting legal cover to people like Warren Jeffs.
If this view is outdated, and our laws against polygamy are causing people real pain, then by all means let them make their case; perhaps times have changed. But we do not make laws in the abstract, and polygamy in the USA is intimately connected to subjugation, abuse, FLDS, and other ugliness.
Since we have denied same-sex marriage on religious or moral grounds, are we saying we should deny the right for a man to marry multiple men or women or a woman to marry multiple men or women because of our beliefs? Isn't that the same reason that we're denying same-sex marriage(s)?
Age verification is OK, since minors aren't recognized to be capable of forming contracts.
[1] Jacob was married to multiple wives.