Mac is the only one in s1 we really get to see have a moment of genuine sweetness, and have a moment outside of the gang where they're happy. and that was the point of introducing Carmen in the first place. To show that Mac is more than the piece of shit he's been up until then. that someone can actually like him. and that he can be sweet. but he'll ruin it.

s3 is the same beat, but worse. he's sweet again, they're happy again, but this time Carmen gets upset because he doesn't want to be seen with her in public, and he tells her flat out: "I'm ashamed of myself." that's not just a repeat of s1, it's an escalation. because now he has the language for what's wrong with him and says it out loud, and still chooses the shame over her. he ruins it again.

it's worth noting that in s3 they were playing around with writing queer Mac but they hadn't fully decided on it yet. so yeah there's definitely queer subtext you can see in the s3 Carmen thing ("I'm ashamed of myself") but the point of that storyline is not a gay guy goes after trans woman for her dick. i think that simplifies it and does the other characters and storyline a disservice