With all due respect, I don't think your English is at fault in here. Like I said, I believe you have a pretty narrow-minded view on how trans folks are. Just the fact that you referred to your so-called trans guy friend as she/
he with the latter in italics, instead of just using the proper pronoun (assuming that they want to be referred as he), makes me believe that you don't really envision trans men as, well, men. It might not even be your intention, but that's what you're projecting with your words, which like I said previously, do come off as offensive.
Masculine or feminine traits don't automatically make someone a man or a woman. Nor do they reflect their sexuality either. The spectrum on sexual orientation and gender is pretty vast and complex. There are no norms, no obligations; there might be, however, people who believe that
x gender and
y sexuality have to conform to certain behaviours, looks, expectations, etc., which is what has made the lives of many LGBTQI+ people so hard to endure even up until this day.
By the way, being trans has absolutely nothing to do with your sexuality, at all. Trans people can like men, women, non-binary people, all of the above. Again, the insulting thing in the case of the game is not for a trans man to be a LI, what's insulting is to believe that's a bad thing.
Lightaces gave you a pretty good overview on how those expectations and such have affected the character of Mason in this context, so there's no need for me to reflect more on that matter.
My honest advice would be not to limit your outlook so much on this. I don't know if your opinion stems from your environment, which has 'forced' you to assume certain things, or if it's something more personal. So, if you're willing, keep an open mind.