Thirteen years ago I watched my wife while over at a mates place, walk over, kneel on the floor in front of my mate, pull his cock out and start giving him oral.

Thirteen years ago, I watched her then stand up, remove her clothes totally, and move forward to straddle his hard bare cock.

Thirteen years ago I watched her ride his cock on the lounge, then watched her have him lie on the ground while she turned around to straddle him once again and ride him reverse.

At that moment, my memory was permanently etched with the vision of her eagerly sliding her pussy up and down another guys cock. It is a vision that I have spent the last thirteen years trying to get her to repeat. It is a vision that I know she is interested in and she knows I am wanting to see. She is also aware that I want to take it further and have the guy cum freely inside her and that I'd even like to try two or three guys joining us and all cumming inside her. This last thought certainly has her interest and causes much more arousal than I think she expected. 

So for now, it's just a case of keeping that thought in her mind and encouraging her to explore it when she's ready....and just continuing to wait until that moment happens again. It will happen, it's just a matter of time.