Maybe Sirius doesn't realise that Harry doesn't
understand Poodletongue.
Also, Sirius is recently escaped from Azkaban
and in a violent state of mind. He hasn't really
calmed down much by the time he confronts Lupin...
having said that, is it just me who thought they
were a couple, before and since? Lupin being
a male teacher at Hogwarts, if anyone has not
read the books but is following our discussion
anyway.