I've always felt that what Sisko did meets the legal definition of "rape by impersonation". Rare in the actual world but still defined and quite illegal. (A real world example would be sleeping with your twin brothers girl friend, her thinking that you're him.) Science fiction, with shape-shifters, alternate universes, mind swapping, etc has many more instances where this could occur. Then there's always that scene in "Revenge of the Nerds".
Well I doubt there's much of a legal system in the Terran rebellion and if there is I'd say Dax pulling a knife and saying she'd kill him if he touched her again covers it.
As for Kira, she'd probably sleep with Sisko knowing full well he was mirror Sisko.. why not she tried it later on too. So I doubt there are any charges there.