This probably feels like a big "owned with facts and logic" moment, but it's symptomatic of someone who has never had to give serious thought to the complexities of human sexuality. In other words, to anyone who isn't heterosexual it just comes off as pretty ignorant.
Emma Portner (who was Elliot Page's wife, by the way, not his girlfriend) is a lesbian because she has done the hard work of figuring out that she is a lesbian. It actually has very little to do with what she does or who she has relationships with, and far more to do with how she feels about herself and her relationships, as well as how she exists socially within a society where lesbians are a category of people, both of which we obviously we don't know. Her sexuality, as an individual, is also allowed to be complicated, because all human sexuality is complicated and its only heterosexuals who refuse to acknowledge that it is.
Transmen can absolutely look feminine, and straight men can find them attractive. This state is usually temporary and transitional (oh wow, it's almost like words mean something), but even in a hypothetical fantasy case where it was not, a straight man who found themselves dating a transman would, at some point, be forced to acknowledge their partner's inner life in order for the relationship to work.
What I think GX has rather underestimated is the sheer and complete lack of regard most straight men actually have for anyone else, especially the people they date.