The diagram of the relationship with Jack, the Doctor and Rose (triangle) being different than Jack's relationships with Gwen and Ianto (V) is definitely part of it. So, I think, is general Gwen loathing, as well as the fact that Gwen's in a relationship (Jack can love more than one person, romantically, they don't necessarily believe she can. Therefore she's fucking Rhys over). Also, they're trying to establish Jack/Gwen and Jack/Ianto at the same time, rather than bringing someone else into an established relationship.

None of that's got a damn thing to do with it the reality of the situation which just is that Jack loves them both, Gwen loves them both, and Ianto-poo's doing just fine thanks.