Geez, what a fascinating bunch of intertwined conversations!
Marten and Faye are so close that it's almost like talking to himself. He can think out loud around her.
Y'know what, I would find that interpretation of it entirely plausible. Or he initially intended to give a short-answer just-the-facts and change the subject version but then could not stop himself from going on about it. We have seen something like it happen to other characters (Sven).
We already had reason to believe that the zipless hookup is not Marten's thing. When Faye and Dora had their moment of "oh, we did not really mean for you to take our advice", instead of their cavalier "that would be boring" the thoughtful answer would have been "but one way to figure out what you want is to go out there and test the available options, not just try to work it out all in your head".
Well, now he has tested an option and found this was not it -- check. Some of us would even say,
"check... now, next option". But Marten being Marten, that's not how he wishes to see himself; Cossette confronted him with how he himself did not pay attention to the personal connection, and that bothers him.