"In a so bad it's good sort of way," Ianto corrects with a laugh. There's no point in moping over their relationship troubles, at least not out in public at a crowded bar - or so he tells himself a bit unsuccessfully. He's here to enjoy himself and he's taking that very seriously.
He also noticed Rose's wrist band in his earlier conversation with her and it hadn't been hard to guess what it meant. He has no way of knowing Jack's thinking about it too, of course. In his case it just makes him regret not pushing Jack harder for what he had really wanted out of this relationship. He'd been too scared of pushing him away, and he still is.