It sounds like the choice is to "repent" and renounce your homosexuality or to let your church take whatever action it pleases.
I'm not Presbyterian, so I don't know how that church government works. But can't you just transfer your letter to another church? I think it's really just a formality, and I doubt that an inclusive church would care whether your former church places a "black mark" next to your name.
That's what I did, to ensure my former pastor had no presumed authority over me, and to ensure that he didn't try to visit me or get me to "repent". I'm sure he knows why I left the church, but I made a point of avoiding the topic in our exchange when I left the church. And to his credit, he had a sense of propriety that kept him from bringing it up.
Of course, my pastor wasn't my father. Can't advise you on that, because I haven't told my father that I'm gay.