I know you're getting a LOT of replies, but I'm going to add a few things.
I knew I wanted to be male for my entire life. Everything I was interested in gravitated towards masculine things. As I grew up, I hoped I grew up to be more masculine. I got excited when I met females that were more masculine, and I got excited because I realized that there's a possibility of growing up to look masculine, despite being born assigned female at birth. My parent knew this, I was talking about it ever since I was 4.
Then, I hit puberty. I was very obviously female. I was very unhappy. "Transgender" was not something i'd even utter to my parents. Back then, I was so sheltered I had no idea it was possible, i thought "transgender" was simply somebody that crossdressed (which i technically already did.)
I had to choke it down, and try my best to just make the best of it. I was female and had no other choice in the matter. I would not transition smoothly, I'm short, I'm curvy, I have big boobs, I have a very feminine face.
Eventually, I said <not allowed> it. I'm not happy this way. As "female" as I can attempt to force myself to be, I am not happy with it, and never will be. I found out I can get hormones (once again, i was sheltered and very out of the loop lol) and started to pursue it. I was 17, and needed parent permission. They refused, and talked me out of it. They said it was so sudden that they don't believe me. I remember thinking, "All those years of awkwardly trying to force myself to be happy with it, and actually put so much effort into trying, and they don't believe me. "
They convinced me to put it off. I kept talking about it. I was told "If you still feel this way when you're 18, we'll support you."
18 came and went, they still kept saying the same thing, mainly because I liked to draw and I dated men. "That's too girly! You'd never do well as a guy." and they convinced me to put it off until i was 21. I tried so hard to be female. I lived as it, I tried to accept the fact that it may never change. However, I still wanted to transition by the time 21 came and went. They still didn't support me, mainly for the same reasons they stated previously. I focused on working on things myself. At age 22, I FINALLY started hormones. Happier now than I ever was.
Others have pointed it out, sexual orientation rarely matches up with gender. Personally, I go off of personality. The kind I go for tends to just be more common in men than women.
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With that said, I've also been in your shoes. I've dated somebody that was a transwoman. I would have never guessed. This was back when I was trying to just be "female."
I thought it was a joke at first. Her body type was as masculine as it gets. REALLY buff, super tall, strong chin, everything.
I felt uncomfortable, but tried to support her. However, that's when she hit me with a lot of curve balls. There were interests she had that I had no interest in, and/or was very uncomfortable with. (shoe shopping, for example. Or wearing dresses)
She also acted more feminine, talked more feminine, etc. She started to get more sensitive.
Eventually, we mutually broke it off. Mostly, we started that relationship expecting hobbies/interests to be the same way as they were when the relationship started. The more I forced myself to do more "girly" stuff with her, the more uncomfortable I got with it. The more I realized "wow this sucks" and I wanted out of it. The more I looked at my own body and wish I could just swap it with her so we could both be happy. We're still great friends, we talk daily, but our interests and personalities have changed enough to the point that they no longer match up well enough to date.
Hopefully you won't have to go through that. I normally don't see it go down that road to be honest. The main problem I had was we had A LOT in common that ended up being things that my partner was forcing herself to do to be more "manly." So when those stopped, our interests were completely different. Her personality never changed. She never changed as a person. Her interests just changed. Since that was the basis of our relationship , it ended. However, we do not need that in order to be good people towards each other, be able to chat, and be able to support each other through our transition. Also, she has transitioned surprisingly nicely. You can only sort of "see" how she COULD have been the other gender if you were looking at her and thinking about it. Most transwomen I meet can pass just fine, but if you think about it too much, you'll subconsciously pick out things that remind you of old traits that can be seen as more feminine or masculine, and it will seem more exaggerated (even if they aren't to other people)
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As a side thing, something I found that would have helped me a lot is to get my partner to write out a list of things they want to change about themselves. Ask "What changes do you want from your transition?" and tackle it on one step at a time. It won't all happen at once, remember that. See if there are any interests that they're forcing themselves to do. See if there are any interests they're holding themselves back from (more likely the case. Met plenty of females, even if they aren't trans, that love football or mechanics just as an example)
You may end up with more interests and hobbies that you can enjoy together.