Considering that they're already in this messed up triad, on his suggestion at that. It's taken some mental gymnastics to work around it but as far as he's concerned, it's been surprisingly smooth sailing and involving more people sounds like a recipe for rocky waters. Better keep it as simple as can be.
"And you'll always be the exception. You know that, Jack." With who Ianto can dally with other than him, the only person around that he'd willingly share with. "What goes on with you two is up to you and if you and I stop messing about, I really have no say in the matter."
He leans forward then, a mischievous twinkle accompanying the dangerous dark tone in his voice. He's kidding, for the most part, but also he's not.
"But if you hurt him? I'll rip your fucking throat out."