Campaign For Liberty — Legalize same-sex marriage

Campaign For Liberty — Legalize same-sex marriage: "A tenth argument is that allowing same-sex marriage will lead to plural marriages or marriages of incest being allowed. But this is, like the fifth argument, a slippery slope fallacy and red herring fallacy. Besides, this argument also argues for no marriage being legal, even opposite-sex marriage, because this can lead people to want other forms of marriage to be legalized."

How is gender significantly different from quantity?

"The issue of whether the state should be involved in the institution of marriage and grant benefits to people based on their marital status or the number of children they are raising is beyond the scope of this article."

Actually if the government didn't get involved then it would completely address your argumument.