at 18 you are considered an adult. therefore allowed to smoke, marry, vote, and join the military.

though, you can actually get married before 18 already if your parents permit.
there are other ways too believe. so there are already special allowances for that.

and the point it while gay marriage is the marriage of consenting adults, it can be used as a valid point in an arguement for polygamy. if a gay man can marry another as consenting adults, why can't a man and 2 wimmins marry if they all want it. also as consenting adults.