The ethical ramifications of such a lawsuit aside, I don't see the problem. The whole "why should gay's be married" thing was about equal rights. Married heterosexual couples have the right, and now these two ladies do, and are exercising that right.
Whether or not this kind of lawsuit is right has no bearing on their "gayness".
EDIT: maybe that last line should read that their gayness has nothing to do with whether that kind of lawsuit is right.