Christians who rightly oppose SSM challenge proponents to draw a line. One counterexample is pedophilia. If you defend homosexual marriage, why not pedophiliac marriage?
I recently ran across an SSM proponent who tried to counter that comparison. He said marriage is a contract, and minors can't enter into contracts.
However, the obvious problem with that counterargument is that adults can and do act on behalf of minors to contract services for minors. Consider parents who contract medical services for their kids. Their underage kids are party to the contract, even though their kids lack the legal capacity to enter into a contract on their own.
Ironically, abortion proponents don't think minors don't need parental consent (or even parental notification).