I am just going to add:
Part of why I have my own view on this subject is from the whole "treat others as you want to be treated" saying I grew up hearing over and over again.
To me, being trans* is a big deal. I'd want a SO to share something that's a big deal, that made a big impact on them with me, and I'd feel cheated if she/he didn't share that with me because they didn't have faith that I'd see them the same way, or still love them. So, if I'd feel cheated by someone hiding something that was a big deal to them from me, it's only fair to consider it being the same for them.
To others here who said being trans* isn't a big deal to them, that's perfectly cool. There's no need to share something that's not a big deal. If I didn't think it was a big deal, and I wasn't afraid of him/her finding out later on, then I wouldn't share.
I also said that I had an experience of my SO both outright lying and hiding something important from me, and I see the result of my SOs bad deeds. I wasn't saying your SO finding out would result in divorce or bad feelings, lots of people find stuff out and become angry for awhile and then work through it. Maybe your SO wouldn't be angry at all, or wouldn't care. Again, it all goes back to your own personal experience, your own judgment, your own way of dealing with things. My experience likely gives me a bias to being more open in relationships.
It's all a matter of opinion and experience. The site is here so everyone can share their opinions and experiences, it wouldn't be as helpful if everyone was too afraid to reply, or is disrespected. And, once again, I am sorry if contributed to you feeling disrespected or not heard.