If a gay couple is married in CT, and moves to TX, their marriage is invalid. so when one of them dies, the Feds could collect taxes. If they stayed in CT, they pay no taxes.
The marriage isn't invalid when a person moves; it just may not be recognized by the new state. But the person is still legally married in a state for federal tax purposes.
Texas' laws on marriage are not going to affect whether the federal government decides to collect taxes from a homosexual couple, as the I.R.S. is a federal agency that is restricted by federal law, not Texas law. So long as the couple is married in a state, the I.R.S. will (now) give them the federal tax benefits of a married couple.
Now whether or not Texas could disregard the marriage for
state tax purposes, I'm not sure. But the basic wording of the Full Faith and Credit clause of the Constitution suggests that they could not do so any more than they could attempt to disregard a heterosexual marriage that occurred in another state.
In regard to whether Texas or another state can disregard a homosexual marriage from another state, my understanding is that, previously, states which had passed a local DOMA constitutional amendment were allowed to disregard homosexual marriages from other states. But now that the federal DOMA has been repealed, I am assuming that state DOMA amendments will be viewed as unconstitutional. Of course, this will require judicial challenges in order to repeal those state amendments, but I assume this is how it will play out.