Jack may be blind to many things, but when it comes to anything sexual he's normally much more aware, and he can't help but notice that moment of hesitation before Ianto returns his kiss, and the feeling that there's something being held in reserve here.

Which means that it's only really a minute or two before he's pulling back slightly, not away, just trying to reduce his own neediness and give Ianto a chance to say no if he needs to.

"Sorry," he mumbles, abruptly very aware of the fact that Ianto and Spike have stayed put while he's moved on and that such advances might actually be off the table now.