Sebastian has such little experience with sleeping, even less so with dreaming - recognising the properties of a dream for what they are doesn't come naturally for him, especially when he's initially caught up in the usual dream logic. There's nothing strange about Constance being here; about them being in this strange location together. Sebastian knows where they are, after all; he's at home here.
"Mm, perhaps something like this?"
One moment later, without Constance seeing quite how it happened, the room has changed around them. It's furnished in 17th Century French style, every piece in the room tasteful and of obvious quality without reaching the opulence of the Court - the colours are warm; the whole room has a homely feel to it. Sebastian knows Constance has been by the Queen's side for quite some time now, but surely she doesn't feel entirely at home surrounded by too much extravagance.