Actually I think that rights, ethics and morals are absolutely applicable to every case. That's what they exist for
Firstly I believe everyone has a right to privacy, even in relationships. There are reasons why having privacy is normally in lists of what makes up a healthy relationship and not having privacy is in the lists of unhealthy or abusive relationships.
That said there are simple factors involved.
Can the CD be sure they will never be found out? No, no-one can be sure, only confidant at best.
Can the CD be sure that they will never want SO participation or to be out or full time? No. They can only be certain of the present and guess at the future.
Is it right for the CD to make choices 'in the best interest of' the SO instead of informing them and letting them make up there own minds. Nope.
Therefore for the best interests of all concerned the most moral and ethical choice is to inform the SO asap... in most circumstances. There are times when that is not always the case and if a CD finds themselves dressing only years after already marrying, if the SO has already expressed transphobic opinions that are unlikely to shift and I'm sure a host of other reasons then it can be right to keep it private.