Everybody knew that when Loki escaped from that state of the art prison cell, made off with that old relic of a staff, and disappeared into the multiverse that he would come back to cause trouble. The question was when and in what form.
Luckily, the genius that is Tony Stark is pretty smart and has some experience dealing with things that aren’t quite what they appear to be. So when he heard that Loki was back, and he found out that the relic was still on him, he took action. He hacked into S.H.I.E.L.D’s database and bought horse tranquilizers.
After a few days he was able to get Loki to let him touch him. At first it was just a few seconds before he’d blink out of existence, but over time they got to know each other and build trust. They had a lot to talk about, too.
Eventually, they started talking about their pasts and what had happened. They both realized that they had a lot to learn from each other. They also both learned that despite all their differences, they were actually very similar. It was at that point that they decided to start dating. Obviously, they were both a little surprised at first, but as time went on they realized that this was exactly what they needed in their lives. The bond between them grew and grew, until it became something much more than they ever imagined.