"He's not my boy toy! He's never been my boy toy, or anyone else's! Stop trying to piss him off! Well, not trying, stop succeeding in pissing him off." On the one hand, he completely understands why Ianto might want to haul off on John after that-- and yeah, Ianto can absolutely handle himself and he shouldn't be overprotective and all that-- but this is John. 'Dangerous' is too mild a word.
The look in John's eyes is very much unexpected, and he's taken aback by it. "That'd depend on how open you actually were."