Do you think it's within the rights of one state to refuse to recognize a marriage granted in another?
Perhaps not, because the full faith and credit clause is implicated when a state refuses to recognize that another state has ordained the marriage. Furthermore, a state cannot deny benefits to out-of-state couples merely on the grounds that they are from another state, under the privileges and immunities clause (of Article IV).
However, that does not mean that a state cannot deny benefits on
other grounds. For example, Alabama cannot say "People wedded in Massachussetts will be denied benefits," but they can say "Same-sex couples 'wedded' anywhere will be denied benefits."
The commerce clause does not authorize the regulation of "anything remotely related to commerce" (which basically means "anything at all," because everything can be said to "have an impact" on interstate commerce). It only authorizes the regulation of commerce itself: "The Congress shall have Power ... To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes."
Marriage may be remotely related to commerce, but it is not commerce.