There are two possible ways to argue this.
On the one hand, we can say that same-sex marriage is not an issue of marriage and federalism. Instead, it is an issue of civil rights. We would not say that states should be able to tell people of different races that they cannot marry. That is not something we would leave up to the states. For the same reason, we can argue, states should not be able to deny same-sex couples the right to marry. From this perspective, this is not an issue of marriage laws, which are rightly left up to the states, but an issue of civil rights, which is not left up to the states.
On the other hand, we can say that this is an issue that should be left to the states. In this view, same-sex marriage is not a fundamental constitutional right. Instead, it is something that should be decided on a state by state basis. What the government should do in this case is to ensure that the stance of one state will not be imposed on any other state. Each state should be free to have its own laws and should not have to honor the laws of other states with different views on the subject.