It is, but the real question really revolves around the legal recognition of marriage. Mainly for tax purposes, but also for employee benefit entitlements, inheritance and so forth. Most of those benefits and entitlements were originally put in place to provide for children, and to protect non working mothers, both while raising their children and later in widowhood. With a homosexual union there can be no question of natural children, and there need be no non-working partner who is rasing those non-existant children.
It's just my impression and opinion, but I get the general impressiion that lesbians want the formal recognition for their relationships, while gays want the medical and other spousal benefits.