Two things (I think. we'll see if this gets longer or is just one thing split up when I'm done typing).
Re: Whether or not fade to black would have bee better: I say no. This bit here, this is basically an extension of the original relationship conversation, and I think, when reading the story as a whole rather than day by day, it flows well and is necessary. The comic wouldn't be
ruined by a fade to black, of course, but whether or not we need to know how physically intimate they get, I think portraying the emotional intimacy and great caring between the characters like this was incredibly important (although I can't say that for certain until I see, oh, the next 8 months or so of comics). To me, this shows a lot about both characters in one comic- Claire's nervousness, both of their affection for each other, Marten's caring (and not caring, about whatever Claire did expect to bother him), etc. Even if it wasn't important, I'd think this was a good comic just for how much it shows in little details, eg Marten's eyes (the expression, not the kind of oddly arted), Claire's bra fumbling and stepping out of her panties awkwardly, body language in general, etc.
That said, while a huge fan of today's comic, not a huge fan of panel 5 (as mentioned), or panel 3 (height proportions what?) art wise. I wish he had re adjusted them just slightly even.
Re: realism: I usually refer to myself as gendersomething as I have issues finding the right thing on the spectrum, but anyway, reference point, I'm not cis, but I'm not transitioning.
Claire's reaction: She was nervous, but she also took things into her own hands. Both seem understandable with a fear of rejection (or perhaps the unknown, or just generally being nervous). Plus, Claire has been pretty strong willed about things (library work, cheating), so her wanting to show Marten herself herself seems to fit for me- and again, this doesn't contradict nervousness, which is pretty apparent from her facial expressions and posture (she's pretty much wincing when she takes off her bottoms). Also, I think it is realistic to a lot of other adult women, wanting to have it on their own terms and, even if they do so nervously say 'hey, this is me.' (which is more relevant in Claire's case).
Marten's reaction: There's a lot less to go off, but anyway- as I said before, this seems like a natural step from the first serious conversation (although I do not think the between dates were filler- while not super useful, they served to show that Marten and Claire had steps in between 'okay, date' and now). Ignoring the details I wrote about 'why is this comic necessary,' since it would all be repetition, I'll leave it at: he has been shown to really like Claire, and also has always been fairly charming, if awkward more early on. Even if he was slightly unsure what he'd see or startled, I'm not surprised that he got over it before really having time to react. Not sure if true to life ever, but I'd definitely call it realistic to his character.
My experience is not entirely relevant, since my partner and I have been together forever and he already was pretty familiar with my body the first time I really had sex with him as a boy. That said, it was really similar in some ways. I was really shakey- how much do I undress, what is he going to do, he's mostly straight, ahhgghg- and the one affirmation I remember completely clearly, was that as things started, he told me that I was a very cute boy. So, on that front, I can relate (although again I disclaim that I know that is very different than this and also that in many ways I am lucky and this is different than many trans or non binary people).
Anyway, I discussed in one of my last posts why I didn't think sex here would be out of question- they both looked passionate, they went behind closed doors, they know each other fairly well by now, there's attraction, Claire has hinted lately, sex can be really nice at stressful times, etc- so I'll set that aside before whether this specific comic's events are. I say yes. While of course verbiage and action are different, Marten made it abundantly clear that whatever Claire's physical situation was, he wanted to try, and I think it's implied that try meant more than a shrug off (eg 'we'll see if it's a deal breaker, or if we're even dating long enough to bang'). Claire seemed very realistically nervous, but both this evening and on past dates it's been hinted at that she'd like to do sexy stuff with Marten. Even if we assume that they haven't dated more than shown at all, or that the timeline is on a shorter side, they've known each other quite a while now as friends and co workers, and there has been a lot of intimacy between them (Claire coming out, the hair scritching, the talk at her mom's house, and today, this). Even if they're not a bang right soon type of couple, that wouldn't exactly be what's happening.
But dammit, I just got sidetracked. I meant to be talking about Claire and Marten's reactions, not more details on the post about whether or not sex was too soon, so sorry.
I'm going to bed, if I accidentally speculated genitalia or talked about sex too much, I'm sorry, but I'm just warning you that I won't see any critique or mod edits 'til morning, and it's not me avoiding it. I don't think I did either of those things, but just in case.