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.
How does the sexual relationship elevate a lesbian family above that of my hypothetical family consisting of two sisters and a child? What is essential to marriage is not the sexual act but the object of the sexual act. Even when the male and female are incapable of reproduction, the object remains reproduction. Where two males or two females copulate that outcome is precluded in every case. It is LIKE bestiality, which is why the term sodomy includes both homosexuality and sex between man and another species of animal. (Ref. Black's Law Dictionary.) So those who were shocked when a Colodado political candidate equates the two OUGHT not have been shocked.
She was only Dr. Johnson kicking the stone.
The personal relationships between man and woman are also different from those between two men or two women, and the mariiage laws reflect this kind of sexual relationship. To attempt to substitute Part A and Partner B in place of man and wife is to make marriage a procrustean bed.