Once again, I guess I had a totally different reaction to this comic than most people. I found it basically a worthless throwaway that doesn't advance plot or characterization.
It's odd, because Jeph said he was moving away from dialogue-heavy strips. Marten gets a ton of dialogue in this strip. But it's basically just a long-winded way of saying "I'm really into you." The strip does not seriously have exposition about Marten's feelings more broadly. He could have said he was exposed to a lot of trans people through his mother as a child, and he never thought about it seriously - until he met Claire and it all changed. Because he sees her as a person first - a person he's crazy about. Instead he talks mush.
This just feels like a retread of their first talk. Claire asks if he's okay, Marten reassures her, and she is beaming at the end. I'm immune to Marten/Claire squee, but I can't even see what's squee worthy in this strip. It's frankly the lamest I've seen in months.
For one thing, a general move away from dialogue-heavy strips doesn't mean that you're going to necessarily abandon them altogether. The art (form) follows, or acts in service to, the story (function). If something can be said with very little dialogue, or none at all, it will (like the strip where Faye's fading in and out of consciousness). If something needs to be told rather than shown, it will be (hence today's talky strip).
Which obliquely leads to your other point. People who have insecurity (or any number of other issues) don't have them constantly. I'm pretty sure that Claire doesn't live under a constant, crushing mountain of self- (or Marten-) doubt, and for him to address it when it's not an issue would be borderline douche-y. But when someone has a valid concern, you address it. She brought it up, he said what needed to be said, she was reassured. That's how it goes sometimes.
It also addresses Isyrion's point:
I just thought of this, the fact that Claire may cause some issues between Marten and Claire. Not because Marten has issues with Claire being trans but Claire, this being her first boyfriend, subconsciously thinking Marten is just humoring her. So far Claire is approaching this right by talking out her concerns, hopefully that continues. (The reason this came to me is one of my best friends is trans and she said she ended up screwing up a good relationship due to worrying about her partner being weirded out by her being trans despite him repeatedly tell her he was cool with it. Eventually the constant worrying drove a wedge between them.)
BTW I think they are a cute couple so I hope this works they both deserve happiness.
I've kinda been there (was dating someone biracial who, for some reason, thought that I was weirded out by the whole thing, even though A: I wasn't and said as much, and B: I'd asked her out in the first place, which isn't something I'd do if I were iffy about someone). I've been on the other side of that situation as well, thinking that something about me was a bigger deal to someone than it was, or thinking that they were a lot less into the relationship than they turned out to be. Communication (as they're doing now) and trust (which they've got, and seem to have been building on) go a really long way.