I actually agree with this. If gays want to get married, good for them, I don't care.
Since you don't care, ignore the arguments against it.
I'm with you. If two adults want to marry each other, why should I care? Laws against gay marriage just give the state more power to interfere in the lives of free citizens.
But only two at a time? Why? On what basis?
Also, would you forbid two brothers marrying? A mother and a daughter over 18? Why or why not would you forbid a father and a daughter over 18 from marrying? On what basis would you forbid it?
Once the definition of marriage is destroyed, there is no basis for forbidding any of the above. They will have to be allowed, on the basis of "fairness."