He hesitates on the answer, not for long but certainly long enough to show that he's not taking this lightly in any way. It's a big, serious, messed up deal and if he'd had his way he would've cut and run when Ianto first called him. As it happens, he couldn't and here they are now.
"Seemed like the only option that made any sense." He shrugs. "Besides... it would make things easier, don't you think? Next time those mistletoe wankers come around."
Yeah, he's been thinking about that and although he's not exactly willing to offer himself up from the get go, it seems inevitable that something will happen. After all, he's felt Jack's erection against his hip more times than Ianto's and wow that's a sentence he never would've guessed would exist in his skull.