While I understand being open to children is a requirement for sacramental marriages it’s not a requirement for civil marriages. Even if children are an impossibility (which isn’t necessarily the case between what was mentioned above and adoption) I don’t think it has much impact on those that have a permissive disposition to same sex marriage.
When you quote a part of what I wrote and then try to relate it to an argument that someone else made, the result is bound to be somewhat confusing. But since you asked, here is my take on the “open to children” issue as it relates to civil marriage.
I think the reason that society has any support for marriage at all is in recognition of the role that marriage plays in bringing new life - children - into the society. This is the general purpose of marriage as a civil institution, even though that purpose is not perfectly realized in the implementation of marriage laws. The fact that some childless marriages get recognized by society is an exception. Such marriages do circumvent the purpose of marriage laws, but not in a way that can be cost-effectively fixed.
There really was no practical way to more narrowly target the laws to achieve what I claim was the purpose of those laws. Consider some potential solutions. For example, only recognize marriages after a child is born. Clearly not practical. If there was no legal pair bonding before child is born, what is to prevent a man from fathering children of two women? And how can a binding commitment be extracted after the fact? No, clearly official recognition of marriage must come before the children arrive.
Well, how about legally recognizing marriage only when it appears children are likely to be the result? This also has problems, especially in more primitive societies where assessment of the potential for child-bearing is little better than guesswork. Some extreme cases could be decided with high probability, like an 80-year old woman. But what incentive does a society have to forbid marriage to 80-year old women? Not very much. For one thing not many 80-year old women are desired as wives. So if a few 80 year old women profit from an institution that was meant to reward child-bearing, the society loses very little from that. And no matter where the line is drawn, it is going to appear somewhat arbitrary.
So on balance societies concluded that the simplest practical marriage law would be to allow and support marriage between any man and woman. But then society recognized that such a law is too simple. It allows certain injustices and undesirable outcomes. Specifically, marriage where one or both partners are under a certain age needed to be restricted to avoid certain obvious abuses. And marriage between people too closely related was also recognized as a bad thing (although primitive societies did not fully understand why, not knowing anything about DNA and inbreeding). So a few restrictions were added. But the overall general purpose of legal recognition remained the same. It is not a societal reward for two people keeping each other company (as good and noble a thing as that may be). It is not a reward for forming an efficient economic unit. It is not a reward for two people loving each other. These things are all nice things and hopefully they are all present in a good marriage. But the promotion of these things is not the purpose of marriage laws.
To the extent that these laws do not perfectly promote child raising, they fail to perfectly achieve their purpose. But they do pretty well, considering the limitations of civil law. They are simple enough for any society to implement, and they do accomplish their purpose most of the time. There being no simple modification to these laws that would make them accomplish their purpose any better, the society tolerates the imperfection.
Now consider adding same-sex marriage. On their own, no homosexual couple can produce a child. So adding same-sex marriage takes an already imperfect law and make it even more imperfect. The purpose of promoting child raising is circumvented even more. But what about adoption, artificial insemination, and surrogacy? Even if we allow this possibility, society can still decide that it is more desirable for children to be raised by a mother and a father. Therefore society can reasonably decide that recognition of gay marriage does not benefit the purpose for which marriage was established to warrant the costs of that recognition.