The thing is that with marriage, there are further protections. A spouse does not have to bear witness against a spouse. This has not yet been tested in the case of a civil union, as in Trammel vs. US. There are a number of other protections granted as a result of marriage. I do not see a reason to exclude homosexuals from these protections.
The lesbian relationship --as a sexual relationship-- does not produce children, whereas the most casual sexual encounter between a man and a woman can.
Really? I never knew that! Seriously, what is the point of that? Are you trying ot address my point that lesbians can have kids? That doesn't exactly do it. You initially said that "marriage is an institution that has developed to provide care for human progeny." Ok, lesbians can have projeny and can take care of them together. Why can't they then get married? Because the child is not biologically one of the parents? Well what about step-fathers? What about artificial insemination?
Marriage is a commitment between a man and a woman for the purpose of creating and rearing children. It has special significance, importance, and value above all other human relationships for that very reason.
In the case of lesbians, the sexual relationship is the very heart of the personal relationship and it is necessarily sterile. The purpose of marriage is to produce as well as nurture progeny. Of course there are families without fathers, or natural fathers. But I cannot see awarding rights to two women joined by sexual desire and not give the same to two sisters living together to take care of a ward or a child of one of them.