Ianto wasn't expecting the kiss, but he smiled against Jack's lips. "Of course we do. You spend all your time someone, chances are good you learn a thing or two. It would be cruel to make you choose. Someone would get their heart broken." He didn't want Martha to be cast aside and he knew she wouldn't want him forgotten either. It wouldn't be fair.
However, since they all got along so well, they could have a bit of fun. Ianto smiled at Martha, before he leaned his head on Jack's shoulder, being careful of his own. This was going to be a sore, fun couple of days.