Reader's Digest version, because I've told this story a bunch of times here already: our first time was a soft swap with another couple we'd been getting increasingly flirty with over the course of the previous couple months. We had actually gotten together to discuss the possibility of a (full swap) foursome in the future. We had a fire pit and some drinks, and after a while that conversation led to some making out/heavy petting, which led to us deciding to take it further "for five minutes, just to see how we feel," which led to us all making out/going down on each other for like an hour and a half in the gazebo in their backyard. We sure felt pretty good afterwards; did it again a few more times, then had that full swap a few weeks later.