Jack smirks a bit because he's won - and because he's rather looking forward to seeing Ianto half-naked and slicked with water later on.
"I should think so!" he agrees, and disappears briefly into the bedroom to grab some Speedos before reaching for Ianto's hand to tug him towards the door. "Come on. I'll change into these when you change into yours!"
In deference to the heat, he leaves his coat on its hook and heads out in his shirtsleeves.