This is indeed a tricky issue, but I do still feel we have a moral obligation to tell, yes, with all of the baggage that this terminology implies. If a person does not want to have a physical relationship with us, then I do not believe that what I am about to say necessarily applies.
In the world in which we live, I would want to know the honest truth about my sexual partner's history. Life and death sometimes depends on it. I'm not saying that a TG person is any more likely to contract a disease than a non-TG. In fact, I suspect (but don't have any statistics one way or the other) that the truth may be just the opposite because of sexual performance issues some of us deal with. However, we still live in a world where honesty in sexual relationships is expected, and is arguably more important than ever. I want to know about my partner and believe my partner has a right to know about me.
Honesty is part of maturity. I will never be ashamed to be a woman. Far from it. And I am the last one who would ever want to put a Trans sign on my head and advertise to the world. Sex, though, is an optional activity. I hope we have learned in our society that it is not shameful, but that it does require a certain level of respect for our partner. If that respect is not returned, whatever may be our history, I hope that we have enough respect for ourselves to walk away from the encounter. Those who want to have sex with a partner as a validation for their own person will invariably become ensnared by that need for outside validation. If we cannot tell the truth to our sexual partners it means that we have already rejected ourselves.
I believe that it is true, especially in the TG world, that we are so afraid of rejection. We want acceptance at all costs, and somewhere we got this idea that secrecy will aid in achieving that acceptance. I know I am guilty of that. But in the end, we have still not been accepted if we let the person believe a lie about us. Some imaginary person has been accepted and we have been cheated of the acceptance that likely could have been ours if we have been truthful. If we do believe (and personally, I do) that a TG woman is every bit a woman, why would we not want to celebrate that with a person we might want to become intimate with? If they walk away (or worse), they are not someone who is worthy of our bodies, especially given the extreme steps some of us must take to make them correct.
In short, if a person accepts herself (or himself) then it should not be impossible to be honest. If we cannot be honest perhaps we are not mature enough to be having sex. It is only within the context of honesty that sex becomes a meaningful expression between two caring individuals.
Peace,

Kristi