Hi, Mike.
My story is quite similar to yours, although I got married in my 40s. Don't be confused by erections and erotic thoughts. That may just be testosterone doing its thing. And it could be your body's way of relieving dysphoria.
You mention underdressing, but not dressing in women's outer clothing. I think it would be a good thing to try that, even if it is only at home. That was what convinced me that there was more going on than erotic fantasies. Yes, they were there (just T doing its thing), but it felt good and "right" wearing a skirt even without the fantasies.
For a long time before I seriously considered transitioning, I thought that, if a person could choose their sexual orientation, I would choose lesbian. Well, duh, the answer should have been obvious at that point, but I continued to think that 2+2=5 for a while.
But, by the time I was ready to do something about it, it was clear to me that I needed transition or bust (pun not intended, but appreciated

). So I went into the therapist's office asking how to go about it.
That is my experience, and yours is likely to be different. All I can give you is my experience; I can't tell you what you should do. But your story sounds a lot like mine, so perhaps mine will help.