Sex happened.
Jack will never forget that first real kiss; the kiss that Ianto initiated while they were still in the SUV. The kiss turned into talking about going home together; something Jack would normally love to do, but he was concerned about Ianto's emotional condition.
They had just stopped an alien threat, visited a hellish slave planet and returned only within the last hour.
Now Jack was normally all for rash decisions made under the influence of adrenaline rushes, but he certainly didn't want to make his tumultuous relationship with Ianto worse with a nightmarish "morning after" incident.
Jack asked "are you sure?" about ten times that night. As they made out, as they drove to Ianto's place, as they stumbled into the flat, shedding clothes as they kept attacking each other's mouths as if each were starving for the other's taste and touch.
Ianto only replied with more kissing, until they were both naked, and the final "Are you sure" was met with a long, drawn out "yeeessssss" as he slid his achingly hard cock against Jack's.
Jack was a switch. He bottomed that night. He had no regrets.
Ianto was a good lover. Considerate, gentle, but firm when need be and definitely good at making sure his partner enjoyed it as much as he did.
And boy did Jack ever enjoy it.
It's always the quiet ones in the suits.
He knew he shouldn't, but Jack fell in love with Ianto that night. The one night that was supposed to be just the one night.
But it wasn't.
...XoXoXo...
They had three more "just-this-once" times before they dropped the pretense of casual all together. There was that one time with the stopwatch, and then there was a vial they found containing a tiny remnant of alien sex pheromones, and then there was a whole day at the office where Jack couldn't stop staring at Ianto's crotch. It turned into hand jobs in the Archives Room.
And as Jack righted his clothes and re-buttoned his trousers, not really caring that there was a small cum-streak on the front; he knew that he and Ianto were getting in over their heads. This was more than either of them bargained for. And neither of them cared.
They kissed, Ianto obviously relaxed and sated as he leaned into his lover's body, totally trusting and open.
Jack said nothing, but he didn't need to say anything. Ianto knew; they were in a relationship. This, what they had been doing together, had formed into an inevitably intimate bond that ran deeper than just fulfilling sexual urges to distract from emotional pain.
Ianto loved Jack. Jack loved Ianto. Regardless of their job and its inherent dangers, it scared them both senseless.
They never talked about it. They were good at that.