I think the entire reason this could work out between Marten & Claire is that he's so chilled around her. No angst, no stress, he just accepts her as she is and enjoys her company as it is, whether that's regular chat over coffee or at work, terrible-cute puns, or more personal moments such as sharing one's sexuality or post-beer back scratches.
Perhaps because Claire trusted him and valued his friendship enough to share such a private matter as her sexuality so early in their friendship, this has deepened the ease he has around her and strengthened their connection (perhaps without him overtly realising). Even if he saw her as just a friend at first, the best relationships often evolve from such a start-point, and for Captain Angst this start point of zero expectations must be a new and refreshingly untroubled way for a relationship to evolve!
Oh, and as for Marten's countenance during The Discussion twixt Claire & ClaireMum, to me it's simply, MMM PANCAKESSSS... The details of his breakfasty presence aren't as important to him as they are to Claire, and ClaireMum is clearly saying nothing new to him, no story, simply the nuts of what happened for him to have ended up there. Claire is the one more angsty about their evolving relationship due to her crush, not Marten. He was probably going to invite her out for pancakes in a chilled way anyway..