Well, the same limitations often apply to siblings and even to parents. Even without civil unions, legal instruments exist to establish common interests . Even when you sign a rental lease together you create legal obligations.
The lesbian relationship --as a sexual relationship-- does not produce children, whereas the most casual sexual encounter between a man and a woman can.
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?