Jack has to choose.
Gwen or Ianto.
Ianto is so softspoken, so sensitive.
Jack's pretty sure Gwen is a slut. She slept with Owen, Rhys, himself, and who knows what went on between Gwen and Andy? She probably wouldn't even care. Jack sort of wants to hurt her for not caring. Hse couldn't help but shag him; he's Jack Harkness afterall.
So, Jack's made his choice. He chosses Ianto, who actually loves him, over Gwen who just wants a shag.
That day, Gwen tells him she loves him. He smiles, tells her to come to his office tonight. He tells her he'll end it with Ianto.
That night, as Ianto kisses him, Gwen sees. As green eyes meet blue, he mouths:
I choose Ianto.
She stumbles backwards, her eyes widening. Jack doesn't see the shattered look on her face, or the tears thar fall.
He never really saw her.