Even for Jack, this really is all about Ianto too. Sure, he always had a bit of a thing for Spike, but he never would have done anything about it (probably, barring warden-related games) if Ianto hadn't opened up the doors to a threeway relationship.
He smiles softly, thinking how very far they've come, the two of them. "You know, surprisingly, I'm kind of glad you didn't. This is so much better."