I had the exact experience a few months ago without the crossdressing part. I was living as a gay man (currently still am since I'm way too early on hormones) and I have a boyfriend who lives with me. He and I have been together for 2 years now, and he's a 100% gay! So of course once I started therapy I knew I had to tell him. I didn't know how to and was afraid he would end things on the spot. So long story short, one day as I was leaving our house to go to work, I was noticeable upset about something completely unrelated and he noticed. He texted me while I was commuting and asked what was wrong, I don't remember exactly what was going through my head but it had nothing to do with my transition, yet I decided this was my chance to tell him. So I said "there's something I need to tell you" and I sent him a cartoon I had found online of a girl coming out of an unzipped male body with the word "#trans" on it. I figured I might as well tell him through text than just keep postponing it. He was surprised but instantly said he would support me, and asked a million questions.
Our conclusion was that he was going to support me and we would remain together as a couple as long as we could, and that if at some point he felt he wasn't attracted to me anymore because of my female appearance, then he would tell me and we'd remain friends. I'm completely OK with that and I understand, I felt it was a great and very realistic response. He reassured, and has kept reassuring many many times since then, that he will support me no matter what and that he loves me.
So we are still together and we don't know what the future has in store for us, but we're both happy with each other. He wants to be involved in the process so he has come with me to therapy, he helped me pick a wig, I dressed up for him at home so he could see what I would look like...Etc. He actually gets jealous if he feels left out of the process.
Hope this helps! If you want to do something similar I can send you the pic I sent to him.
