The thing I see with Jack is that he flirts with people. Lots of people. And the message he's putting out is "I think you're sexy and if the feeling's mutual I would like to have sex with you. Do you feel the same way about me?" If the answer isn't a clear yes, he accepts that without nagging or pushing. He'll still happily keep on flirting if the answer he gets is "Sex no, flirt yes", but there's no pressure in it.

So flirty and promiscuous, but never one to take advantage. I'm quite sure from what I see on screen that he could try gently with Ianto the day after Fragments to see whether he was reading things correctly, get a less than unequivocal reaction, and back down to flirt-only.