Do the states get a say in whether up is down? Of course not. Some things are simply reality, and are not within the purview of sane legislation, though I realize the power-mad legislators of our era claim god-like ability to define reality itself.
You propose letting the definition of marriage itself as something to be thrown open to variable legislation, yet you fail to indicate a single state which would not advocate the language of the marriage amendment you seem to oppose.
At some point you must recognize you oppose popular government in practice, unless you can cite an actual state which is constrained by this legislative act. The people of all 50 states call for action. It is not in any one of their interests to stand in the way in behest of a theoretical 51st.
The "gay marriage" movement thus far had avoided any legislative plebiscite. Either this is an issue upon which the polular will matters, or it is an issue of inherent rights. Neither of these is addressed by appealing to a federalist priciple - particularly in a case in which all 50 states are in popular agreement.
Our Constitution allows the nation to legislate via amendment when there is broad consensus among the states. Do you reject this in principle? Or do you have some special knowledge about this specific issue which calls you to object on that basis instead?