[ Kirk let's him get things out at his own pace, sipping quieter and occasionally taking a bite of pie. His eyebrows raise at the mention of Spike, but again doesn't interrupt. When the other finishes speaking he is quiet, turning over Ianto's concerns and weighing his own words. The silence is filled by the sound of his fingers tapping against his mug. ]
What I'm about to say is in no way an accusation or placing blame. But you say you have problems communicating your concerns with Jack sleeping with others, that it makes feel like you're somehow lacking. But you've also admitted that you slept with Spike - who suggested I actually meet Jack by the way. And you've carried on the relationship with a Spike and Jack at the same time.
I think you should consider why you went to a Spike in the first place. Why you continue with him. It's not because you don't love or care about Jack - you clearly do. But it also seems like Spike fulfills you in a way different from the way Jack fulfills you. This doesn't mean Jack is lacking. It just means he and Spike offer you something unique in their own ways. And maybe, like you, Jack believes he too is lacking when that's not true.
If it were me - I would think this way. I would want you to be fulfilled and recognize that I can't possible do so in every way. No one can. That's the nature of being - well, a living creature. You love a person for what they can offer you, and accept their flaws because that's who they are. So it's not that you lack anything, Ianto - he cares very much about you, even I can see that. It's just that sometimes he - men like him and myself - we need something... other.
I know it probably counts for shit all coming from me, but that's how I see it. Just seems like neither of you has gotten around to vocalizing that.