Honestly, I don't think they're having sex. If they are, it's going to be pretty passive on Marten's part, I think.
It's been hinted for a while that Claire wants to push the relationship in that direction (but has anxiety about whether it's too fast), but I don't think Marten is in the mood for sex right now. (Note that I didn't say that he's not in the mood for sex with Claire right now, but that he's not in the mood for it period.) He's exhausted both physically and emotionally, after all. Claire's good at picking up on this sort of thing, too, and I think she'll hold back upon noticing it.
Now, in the morning, when he wakes up, with Claire snuggled tightly against him, awake but not wanting to let him go? (Or, after she makes breakfast for him, and then returns to bed.)
Pretty sure that's when the Arbor Day 2003 banner can be brought out. (Also, to be honest, I think they're going to want to talk about things, and that conversation might happen while they're doing those things, but it'll need Marten to be alert, I think.)