Sorry about the night on the couch... But
I'd not totally give up, but please tell me you didn't ask her if she'd like to fuck the neighbours... My advice would have been way slower... Talk to her about the fantasy first, if she's excited in that at all take it to the next level. Now, I honestly think you'd still have ended up on the couch :slam"
Think of it as babysteps. When I first told my wife about my threesome fantasy she left me (we were still dating). I convinced her to give me another try, and when we talked about the fantasy again it ended up in a big fight, at least she didn't leave me that time, since we were married... You're getting my drift.
When we started swinging, she was the initiator
Now, don't pursue this if the message was a clear: "No way..." Let it be and come back to fight another day... Babysteps... And most importantly, you're marraige is more important than swinging, always remember that...