Good discussion. This topic does come up often but I am surprised at the new responders and the lack of conflict. There has been some conflict but it has been worse in previous threads.
Ok, I agree that it is not an obligation. Not at all. Would I disclose? I want to think I would. Do I disclose? Well, sort of. I told my wife well before transition started. Until I did, she thought I was cis. Clearly I could not transition and live with her without her noticing.
I also told a previous potential life partner and that disclosure ended our relationship leaving emotional scars that have faded all too slowly. Personal harm is not the only fear. There is also the risk of heartbreak
That said, most pre-transition trans-folk who are in relationships would not be faced with such a question. If a trans-male presents as a cis-male this is almost a non-issue in the eyes of society. There would be no expected obligatory disclosures.
So pre transition, it is not required. It is fairly obvious during transition (especially pre OP) depending on well, being clothed or not, and presentability.
So... when this question comes up, it seems to be primarily a post OP issue. That or a pre-OP issue if someone passes very well. In either case, there is no obligation.
There are a couple reasons why I encourage people to disclose this information. These have been touched upon already so I hope to find some new nuances to add to the discussion.
First, it potentially opens up your ENTIRE life to talk about your being trans with your partner. Failure to disclose being trans limits the stories you can tell... assuming they are fine with it. Sure many of us wish we were cis women but we aren't. I would find it odd if my partner avoided talking about the first 18, 28, 35 years of their life and I would worry about the intelligence of my partner if they did not share those feelings.
Second, as a lesbian and a former straight male, I can sympathize with the potential betrayal one might feel if they were never told, or told after intimacy, kissing, touching... etc. I would have never resorted to violence if the rug had been pulled out from under me, but it really could have messed me up... and I have always been TRANS. Sure, with age my sexual preferences have opened and matured like a fine wine, but there was a time when I saw things in black and white, and back then, penises were entirely gross, I only was attracted to cis women, even post OP was off limits to my sexual desires. These things did not interest me. They did not turn me on. I had no problem with others being into men and trans folk, and I enjoyed platonic friendships with men and trans folk... but sexual attraction is sexual attraction, and part of attraction is physical, especially when young and male. I just don't find it fair, if you are looking for something long term and intimate. The earlier, the better.
Now, I know this is not a simple issue and there are legitimate reasons to stay stealth in some situations. In generalized terms, it is usually a bad thing for the relationship. It is a huge elephant in the closet, and in this day and age, with social media finding newer and newer ways to bring one's past back to haunt them, it is becoming harder and harder for people to avoid their former life entirely. If you are in a relationship with a good person, they still may feel betrayed or find their attraction to you reduces greatly, they may even be grossed out. If you are with a bad person... well...
The chances of a potential person just going, "Oh, OK, cool." after you failed to tell them for a length of time are not that great, no matter how much we wish they were. And, the longer the relation lasts, the greater the chances they will find out. The later they learn, the greater the risk of betrayal.
This really is one of those YMMV things, but I thought I would finally chime in on one of these threads.