the way that in s1e1 Charlie calls Mac an asshole and then all he is for the rest of the season is an asshole. Then he meets Carmen and we see a different side of Mac actually giddy and happy and having fun all day. and the point of introducing her is to show that Mac isn't just a scumbag, he has this capacity to be really sweet and make someone happy outside of the gang. but Mac ruins it because ultimately he'll always choose his shame and himself. (and then he goes back to the bar and Dee punches him in the face to drive the point home).
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
the point is that Mac ruins everything because of his bigotry. and shame. and there's more to him than being a scumbag asshole, but he'll never get there
