Most likely, an amorous encounter in which he found out he had a penis.
Like what happened to Danny Bonoduce years ago. He just threw the guy an a$$ kicking.
They need to start a transgender pride movement or, barring that, a legal requirement for transgenders to disclose that they are transgender, like men are expected to get consent from women in every step of the relationship. Either way, the element of deception would be gone and they would be a lot safer. So, transgenders, proudly announce your transgender istatus and live another day.