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.
As you have hinted, marriage is a sexual relationship. A relationship between two sisters taking care of a ward is not sexual. I have know two lesbians who plan on having children one day. Both of them have a similar story; they want to settle down with a woman who they love, and then have a child. The only difference is the origin of the child, and that does not sound like a good reason for disallowing the union.
Suppose you have a kid raised by a lesbian couple who gets divorced. Would it be right for one of the women to deny visitation to the other? Furthermore, do you not think that the child would be devastated at the loss of a parent, even if that parent was not biologically his or hers, and that both parents were of the same gender? Or, suppose one women decided to not pay child support?
I suppose there are ways to contractually get around many of these things, but that still seems very much like a "separate but equal" policy, and that is not right. Especially since their union will never be equal to marriage, because there are things like the fifth ammendment extending to marriage, and because the rights and protections of marriage are subject to change and if things are turning sour in the relationship, one member may not want to sign off on a contract covering this new protection.