This is a clear as mud gray area where either side can be argued convincingly, so I would say mostly it comes down to what each person feels is right.
There are some facts though that you just can't really get around. The biggest one being- in our current cultural climate many people (if not most) would find it perfectly reasonable for somebody to react in horror to the news a partner, past or present, is trans. As long as this is true, I think it's a little selfish and unfair to withhold this information from a partner, even though I understand as well as anybody a FTM
was actually born male b/c they were born w/ a male brain. I get it, it's just the world doesn't. I think if you want the moral leeway to withhold this information, then you need to get out there an help change this perception. Be visible, be normal, be kind to others

. These things will help.
We could really use the equiv of Queer Eye for the Straight Guy, how much good did that show do for the gay community?

I understand the appeal of stealth, but it doesn't help the cause when the most normal and least "threatening" of our community disappear into the woodwork and the freaks who went on Springer are pretty much all we have left for a public face. I see this slowly starting to change is the good news, but at this pace it will be generations before we are even accepted to the extent gays are today.