He doesn't assume. It could very well be one room for Iskandar and Waver shares a room with Kirk. He doesn't necessarily think that is the case considering Iskandar and Kirk are lovers... Alright his own argument is falling apart around him, because he does assume the two of them are lovers and it would make more sense to split up with that in mind.

Now he feels bad for Waver.

"You actually have tea down here?" So far he's seen alcohol... or what he assumes is alcohol. Maybe water?