Some great responses.
Reine you raise a very interesting question about degree and in my opinion is THE key question and the biggest problem is that the degree can and will often change.
So take myself as an example from 12 to 40 a very occasional CD who dressed for a turn on... it would probably not have been a big deal if I had shared my 'kink' with my SO if our relationship was one where we were intimate and shared our fantasies etc... I could have truthfully told her I was not wanting to dress to attract men, nor was a dressing because I wanted to be a woman in any way.
BUT then at 40 when it all changed for me very suddenly, so when Becky emerged and I have some level of GD.. What then? Do I update my SO and say actually now its more, that is a VERY different conversation. Of course that is the conversation I would have to have if I intended transitioning, but what if my feelings were somewhat in the middle? What do I tell my wife? That I think maybe I am kind of a trans person maybe? Imagine her unease and uncertainty about her/our future when I can't even tell her what I want?
Although I had some pretty intense feelings and a level of GD, there were external factors that caused me to delay telling my wife and then Becky left me for over 3 years. Imagine the situation that could have occurred if I had told her everything and then lost Becky.
I think that telling becomes very obvious when one is at either end of the spectrum but is far more complicated when one is in the middle.