I have several times. Two are particularly memorable.
The first was still in transition. I would see him occasionally, when he would ask if he could practice "suppressing his gag reflex." (It did need work, I would often make him puke.) It was interesting watching the transformation between the times I saw him.
The other was a complete surprise-- I was at a private sex party in LA, and saw this very cute guy working as part of the event staff. Compact, somewhat muscular, soft hair all over his arms, and a lovely long brown beard. I made a note to find him later when he was free. Sure enough, at the end of the night, when the crowd had largely thinned out, I bumped into him in the dark in back. He says "Could you fuck me with that?" I replied that I'd been waiting all night. I was getting a condom ready, and didn't really notice as he slipped off his shorts and got on his back. But when I lifted up his boots and put them on my shoulders, and looked down, there was...a bit of a surprise! I confirmed he wanted it in the front door. I said "Give me a minute." It was late, I was tired, and I didn't think this was going to work out, but I surprised myself by becoming quite hard. So in I went. I got in deep, and felt what, from what I had read, I assumed was the cervix. I started to gently massage it gently with my cockhead, and then HOLY SHIT! No one's ass ever did that to me! These straight people might be on to something... It was actually a pretty good fuck. I didn't cum, but he certainly did.
Actually, FTMs are far more common than many people think, and I've encountered a number of others both sexually and socially. Some of them end up with almost exaggerated male appearance, so you would never guess. I remember this one beefy, hairy, musclebear I would see in social settings sometimes. I had always noticed that his hands, and particularly feet, seemed very small for such a guy (Then again, I have such ridiculously big feet, that everyone's look small...) Sure enough, that was the giveaway, he did eventually tell me that he was biologically female.
