Apparently in Asgard, everyone kisses everyone. So when he comes to Earth, he spends time with the Avengers, and starts kissing them too. At first, they're kind of weirded out, but eventually it just becomes normal and they start doing it too. I'm pretty sure that at one point, Tony walked into the kitchen in the morning all tired and groggy, fell, Clint or Bruce caught him, and Tony kissed him in thanks.
So yeah, I know that's not a lot to work on, but if anyone knows the fic or one like it, it'd be very much appreciated.