“many states don’t allow civil unions.”
True, and in those states, neither gays, nor straights, nor polyamorists, nor cousins, nor polygamists, nor celibates, can enter into a civil union.
It’s not just gays who can’t enter into a civil union in those states. The law applies to all of us.
In states where homosexual marriage is outlawed, I can’t marry another woman. Or my brother. I can’t marry a man who already has a wife, either.
The laws applies to all of us, equally.
Also using your logic, women can only marry men and men can only marry women. It’s a classic equal protection violation. Of course, we both know these are very silly arguments that obscure the real point.