Ianto's assurance is persistent enough that it sends a shiver of arousal, of love, through him because it's a big thing to promise, even if Jack would never abuse it. Not intentionally, at least. He's not always quite as careful with other people's feelings as he'd like to think he is.
For now, though, he's absolutely certain they're on the same page. He's groaning unreservedly as he feels Ianto swallow him up, and Ianto's moaning beneath him, hot and tight and amazing. Once he's in, he lets go of his cock and gets his hand on the desk instead so that he's got all the leverage he needs to start rocking into Ianto, slowly at first, and then steadily gaining pace, every thrust perfectly angled because he knows exactly how Ianto likes it.
And meanwhile his mouth is on Ianto's back, nipping at him through his shirt, sucking little wet patches in the fabric that he'd have to hide under his jacket afterwards while they made his shirt stick to him, if there were anyone to hide them from any more.