I agree with you, if we're talking about gender-nonconformity (CDing). This does not involve a wife altering her sexual preference to follow her husband's cross-sex transition.
But, even in the best marriages, a GG may not be able to become a lesbian wife to a transitioning transwoman ... which compares to a husband who would not be able to become a gay man if his wife were a transitioning FtM. It's not just a matter of having to alter one's sexual preference (which in my view is hard-wired), it is also living with the stigma (still) of being in a same-sex relationship which would be an even greater challenge if the wife is hetero.