This is a question I have asked many times in other ways without any direct response…
I thought I already answered that one for you. Society has decided, for reasons that seemed good to the members of that society, that the nurturing of children was important to the future of that society, and that such nurturing should best be done by the biological parents. The society has not decided to make all babies in test tubes and give them out to people that some bureaucrat thinks is the best nurturer. Society has not decided to take all babies from their biological parents and give them to others based on similar criteria. The ideal is that children should be raised by their biological parents. Now if you have an argument why this ideal should be changed to one of the other two I mentioned, or some other ideal, then I would be interested to hear your case for such an ideal. Until some better ideal comes along, the decision made by society to encourage children to be raised by their biological parents stands as justified by the decision of society.
Of course their are exceptions to this ideal. Some children are orphaned when their parents die. Some children are created through artificial means. Some children find themselves under the care of parents who are so bad that the society steps in and takes them away from those parents for their protection. And some children are conceived through rape, or without parents in a committed relationship. All of these children need to be nurtured by alternate means, such as adoption, or the care of a single parent. But all of these exceptions to the ideal case do not negate the value of keeping the ideal case as the goal. At least that has been the value judgment of society.
Now that we understand what society has decided is the ideal nurturing arrangement, how does society go about promoting that ideal? The institution of marriage is not a perfect implementation of that ideal. As you point out, some people that are married cannot or choose not to have children. But if you try to come up with a better implementation, you will see there are many difficulties.
If the recognition of marriage were limited to only those times when a couple was nurturing a child, that recognition would run out when the child became an adult. Couples anticipating marriage would anticipate this consequence and take it into account in their decision making. Marriage would not look so good to them. So the encouraging effect of the institution would be diminished.
There are also practical difficulties to making marriage more narrowly targeted. Couples might have to pass fertility exam before being allowed to marry. Such an exam is likely to be highly inaccurate.
About the only limitation that I can imagine being both practical and not too harmful to the job of encouraging the ideal nurturing situation is to disallow entry into marriage for couples who are both in their 80s. It is an easy criterion to administer. But it would apply to so few couples that is not worth enacting such a limitation.
So if we look at your objections, they do indeed show that the societal implementation of marriage does not perfectly accomplish the goal of fostering the ideal child-raising environment. So what is your solution? You want to change the society implementation of marriage so that it is even worse at accomplishing the stated goal. It seems that is a wrong-headed reaction to the problem you posed.
If you reality think that gay people should be given the benefit of marriage just because some of them end up taking care of children, then why not give equivalent benefits to single caregivers of children? And why not limit gay marriage to only those couples who actually do take care of children? You see, the problem you posed cuts both ways. If child-raising is not a sufficient basis to grant marriage status to heterosexual couples, then it is also not a sufficient basis to grant that status to gay couples. The only argument that I see as really a good one for you to make is to say that societal marriage is worthless, so let’s give it to everyone.