Personally I tend to think the writers rushed too much on trying to establish Jack and romantic relationships with Ianto, Gwen, and even John.

By giving him 3 ongoing love interests they limited the stories they could tell. They can't do anything like CJH, or even his relationship with Estelle in Small Worlds, without making him seem like an insensitive opportunistic bastard.

If they wanted to do a one-episode romance, or even a 5-episode story arc with romance, their future choices are very limited.