Oh I agree, I actually hated that scene, it makes me cringe. Although moments like that and the way he reacted to Donna in DW, show what a good actor JB is, and it is one consistency in his characterisation. Jack flirts for a purpose, gets people on side, or gets something else from them, such as he tried with Rose. He does do a good job of showing when he genuinely has no interest.

The main problems with that scene, the consistantly awful acting of FA, and the fact that in fact the only other person there who had snogged Jack was Ianto. Everyone else hadn't had a go. So it was redundant.