Well one could presume that Lisa's definite death by Jack would have frozen a tiny bit any relationship whatever they have on going.

When you think about Ianto's level of treason (and IMHO, Jack got one hell of a bitch slap and maybe realized that Ianto more or less turned him into a rapist since at that point it's all fake for Lisa's sake for our captain's POV), for Jack to be so happy (and boy he couldn't even wait the end of the day he had to get ride NOW of the others) to resume their affair, it's either Ianto is SPECTACULAR in bed or Jack's seriously hooked.

My humble opinion anyway ♥