I found the question rather appealing, because for me it touches on: a) my favourite subject and, b) isn't necessarily about traditional fantasy, but alludes to a moment or opportunity that was there at one time and passed.
For me the answer is a bit of a Part One and Part Two response. There are two equally lust-worthy thoughts that come to mind when thinking of this.
In my experience, I have had come very close to fulfilling my desire to have a threesome a few different times. When my husband and I first dabbled with the idea of swinging, we met a couple that became good friends and shared our willingness to explore and experiment. The wife in the couple was the subject of my Friday blog.
And while we played around with this couple, we had all agreed that taking the fun to a full-swing situation was not where we were at emotionally at the time. We still had a lot of curious thoughts we needed to work through without pushing the limits too quickly. Fun is fun, but not at the expense of regret and damage to a marriage just because the alcohol-induced courage is a little higher when sexually stimulated.
Along those lines of achieving my desire for a MFM also sits the more recent Hotel Rendezvous with my Office Guy. The stage was definitely set for some incredible sexual adventure, but again the moment needed to develop in a nature way. My husband and I had agreed that, in the heat of the moment should my experience with my Office Guy reach the point of no return and having him give me a good, hard fuck, I would be allowed to enjoy it all - as long as I followed the appropriate rules of safe sex. In hindsight, I'm not sure how capable I would have been at rolling a condom onto his hard cock ... but everything played out in such a way that the oral sex we shared proved to be the pinnacle of the experience ... and hopefully opened the door for more exploring down the road.
The second part that comes to mind in this question goes back to the time when I was so close to answering my bisexual curiosities, but couldn't because my lustful partner and I had indulged just a lot too much that night. If you are a long time reader of mine, you will remember the blog about the weekend I spent at a work-related conference with a female coworker. At that time in my life I was just beginning to explore my sexuality in a bigger way. My husband had encouraged me to be more open about my desires, my fantasies, my sexuality and I was definitely enjoying where the path was leading me.
The details of the whole experience can be found in this entry - Bisexuality | When It All Came Out. But to finish my thoughts here, the moment I would love to have back is the chance to complete that flirtatious and tormenting evening with what I really wanted to do: fulfill the opportunity for a passionate night with her; no husbands, just her and I alone naked and not as drunk as we were on the night when it could have all come together.