I don’t necessarily think that Dennis is trans but I do think it’s interesting to consider how much self loathing a mtf Dennis would have just from watching Dee grow up
Dennis got to be the perfect golden boy (later golden god) who had all his mother’s affection while his twin sister was always lesser. Dee’s own parents called her ugly, thought that she wasn’t special or successful. We know Frank hit her, although it’s unclear whether he hit Dennis.
Dennis watched Dee at school get bullied relentlessly for a difference that she couldn’t control. She wasn’t pretty enough at school, either. She wasn’t that smart or that strong because she was a girl (a thing that I’m extrapolating just from how they talk to her as adults, but also feels in line with the vibes of the time)
Even if Dennis did feel like Denise (is that funny?) on the inside, why would he want to let that happen? The closest girl in his life is his twin sister, the person he literally shared a womb with, and all that the world has Ever told her is that she is not good enough, will never be good enough, and so much of that comes either directly because she’s a girl or can at least be viewed through that lens
Why would he ever allow himself to be a girl when all that he wants in life is to be seen as perfect, and the closest girl to him has always been seen as basically the opposite of that?