Oh God, believe me if she did pop up NO ONE would be cheating, but sensing someone's feelings without the context could lead to misinterpritation.

Ianto wouldn't get between them, he loves Jack enough to know he was never really happy before, and that he is now. He's happy as he is for the moment, but I might get him interested in someone soon. He/she would have to fit in on the TARDIS though if we don't want Ianto to leave.