The parallel sex scenes could just be a stylistic choice rather than anything metaphorical.
I really think this is the case. For it to be more thematic and narratively fulfilling, I think the crucial missing piece is that Marina doesn't have the same "weight" to the story as Andrew does. This comic has been 95% from Andrew's viewpoint, and the mere handful of times it's been from Marina's solo viewpoint has been as part of a flashback retelling.
Contrast that with other "Mirror Characters". The opening to Hobbs and Shaw is a perfect example:
Our two protagonists are mirrors to one another and we have back to back (or even simultaneous scenes) play out to show that. It furthers our understanding of each of the characters and who they are.
Or the Superman/Batman comic series (perfectly parodied in ShortPacked):
From a character standpoint, having things play out back to back show us their crucial differences or crucial similarity. From a narrative standpoint, it can show a character's arc playing out and whether that converges or diverges.
But this chapter in Mirror isn't a culmination of respective arcs for both of the characters. If we take this chapter in a vacuum, you could some symbolism. Looking
just at this chapter, you could make the argument "This married couple gave into temptation and when given the chance, they chose to cheat on their spouse". And so it could be a powerful moment that solidifies that "it's over" for them. But the problem is that this chapter isn't in a vacuum. Elena has
already slept with Nikos, so the two characters aren't really at similar points in their arcs. So the comparison doesn't really work.
Really, we
already had a more coherent contrast between the two much earlier on. If we look at arcs: When Andrew was propositioned by Nikos, he (repeatedly) let their encounters become increasingly sexual. We can contrast that to Marina's responses when being propositioned by James. She rejected him. And when he showed up unexpectedly again, she's consistently been trying to keep the relationship from becoming intimate, and only giving ground when absolutely necessary. That's very different from Andrew's arc, where his protests are internal or non-existent.
Nikos may have selfishly shaped Elena into exactly who he wants her to be.
As stated above, this scene might prove very pivotal in learning why Elena becomes who she is:
I actually think Stevedore100 and you are entirely correct here. It seems entirely likely Elena is more of a creation of Nikos than she is of Andrew. Prior to the memory loss, "Elena" doesn't really behave anything like post-surgery Elena.
What if “straight, cisgender, happily married” Andrew was a character the whole time? I think it’s possible (given the rest of the story) that we get a big twist like that.
If he truly felt forced to marry Marina, it makes sense that he may have method acted his way into being a good husband for her- very similarly to how he transformed to be Nikos’ good wife.
I would argue, if that's the twist: it ends up being of no consequence to the rest of the story in terms of actually effecting the plot. We already know that the mechanism for Andrew acting like Elena is "method acting". Andrew also being an act doesn't give us greater insight into how that works because we already knew that's what's happening.
On the other hand, the other question we still have: Why is Andrew being feminized? isn't really effected by Andrew not being "real" either.
For this twist to matter to the plot, it would require the "villain" of the story to have found out Andrew had this "method acting problem", maybe by knowing the "real" Andrew (who would be going by a different name), and realizing "Andrew" doesn't really exist. And therefore realize this would be an excellent opportunity to feminize him because he has this unique problem. But that would require someone knowing Andrew before hand, meeting Andrew again after he hooks up with Marina, and then deciding "You know how I'm going to take advantage of this unique situation? Let's turn Andrew into a Greek milf", and have that benefit them in some way.
I think at this point, introducing a twist like that will require way too much info dumping to be narratively satisfying. Because for that twist to work, we'd have to have a reason why this original personality let the Andrew personality get them feminized.
All that said: I can see the "opposite" being a possible twist. Rather than Andrew also being a performance, if Aphrodite was the third "character". That I could see: where the Aphrodite personality is "maturing" the Elena personality so that the Aphrodite can get the body she wants.
That could be an explanation for why the various blackouts: Aphrodite took over and is developing the Elena personality (which you can think of as either a separate personality, or she's "locking" the Andrew personality with Elena's traits).
BTW, the reason why I don't think "Aphrodite" can be the "original" personality, and Andrew as the acted one, is because if that was true, there's no reason why "Aphrodite" wouldn't have started transitioning already before the creation of the Andrew persona.
I'll also note, despite how some of that could logically work, I do think the mechanisms shown don't corroborate that line of thinking. The story has overall tried not to call it a split personality, even though it's almost easier to describe it as such. So far, there's only been one explicit time we've seen where Andrew has "lost" time, and that's during the timeskip.
Let's recall that the other times that Andrew has "lost control", we can see that with the inner thought bubbles that he's still Andrew. Take for instance:
By itself, it can be a AHA! This was when another personality took over! It was hidden in plain sight!
But if we go back and check, this was "Elena's" response at the time to being kissed after being drugged.
That's not another personality that switched into being control. Elena is responding like Andrew. Now Andrew does pass out after this, but "Elena" committed to staying as Elena
before passing out:
This goes back to the whole "Nikos is a bastard" thing: Andrew didn't quite realized he was effectively committing to this charade for longer when he says this line. Blame it on the drugs, the alcohol, or maybe even just Andrew not being particularly strategic. Frankly, if I was in that situation, I think I'd have said something similar, because if you really read what Elena says here, it doesn't come across to me as "I intend to be around all the time." She's being hospitable. "Hosting Joel" can literally be as minimal as making sure he has a nice hotel room, and access to a driver or something. This is the lowest of commitments.
Just about every problem that has moved Andrew's feminization forward is due to Nikos. Now we don't have a clear motivation yet ("how/why the hell did this plan even get made"), and the "clear things up" chapter seems to be trying to exonerate Nikos, but I genuinely have a hard time buying it. Maybe the clearing up was only meant as an explanation as to why Nikos acted like Andrew never existed when confronted after the timeskip, but that's nowhere sufficient to explain the rest of the weird shit he's doing/not doing.