That argument is an absolute joke, and you know it. Gays are consenting adults, so let's stop all the bull about kids and pets, shall we? And since gays are not free to marry the person they love (such person being of the opposite sex by definition)they do not enjoy the same rights as heterosexuals under any rational definition.
Marriage is not a right, any more than driving a car is. You have to qualify. Gay people do not qualify, and by definition they should not qualify. Their issue now is that they are trying to change the standard for their own benefit.
I don't know how to answer you in the legalise you seem to crave. If it is discrimination, it is necessary to protect the institution of marriage.