Lechner v. State, 715 N.E.2d 1285 – CourtListener.com
here is a case where a defendant won an appeal for sleeping with an underage person and won. But yes such cases are rare.
The difference between the transgender question and underage is that it isnt illegal to sleep with transgender people. Given that there is no intolerance laws (gladly there is; gladly there isnt) the legal aspects would be different.
Even though it was my assertion that has forwarded this conversation on legal matters, this is probably not the right part of the forum for that type of discussion. I apologize for unwittingly derailing this thread...
This being the dating and relationships forum I would like to try to go that direction. Ignoring the legal aspects that (I brought up) the relationship aspect of the op, seems a bit easier to tackle. It boils down to individual sexual orientation. As stated previously we all have our own preferences and reasons why we chose a partner. Some people do not want to be romantically involved with transgender people, and some who do. And some might not have, but given the new information their love transcends their previous position.
Obviously my personal position would not be acceptance. Regardless any further romantic relationship would be over. Continuing a friendship would be tough given the level of distrust their actions created. Lets pretend that the secret was not transgenderism but that they were a fugitive, and they only told you after they were arrested. Again its a breach of trust. In romantic relationships I put a lot of value in trust. I have been married for 22 years, the only that happens is with trust and understanding. The transition from one sex to another is a huge part of what makes that person who they are. I seriously doubt that anyone who would hide such a thing can be sincere in a relationship.