No, I agree that it's mostly the whoredom issue: that's where the tears of shame and humiliation come from, at the end of that scene when he's offered the job.

But the quick getaway --"I should go"-- seems like not the Moment He Realizes but the Moment It Leaves the Realm of the Theoretical. GDL says (and I take his word for it) that Jack is Ianto's first man. Which, taken unaware as he was, and embroiled in an economy-size heartache as he was, would take some mental adaptation.