I think it has been a combination of factors. Sex drive varies over the years, and my wife had a strong sex drive early in our marriage. There was something of a lull when she had children, It returned pretty strongly a few years later, and at my urging she was willing to experiment with other men--two of my long-time friends.
The first friend was something of a selfish lover, always tending to his own needs, but less so to hers. Still, there was something about being used as his little fuck-toy that struck a chord with her. Frankly, I was better hung, had better endurance, and always made sure she came when we were making love, though more often from giving her oral gratification than coitus. Yet there was that feeling she was being pursued and my first friend's unabashed approach to making her submit to his needs that appealed to her. It wasn't a ...... fantasy per se, but it had that element of having to surrender to his carnal desires.
The second friend was another matter. His cock was about the same length as mine (eight inches or so), but half again as thick--think beer can. He also had great stamina, and really worked at making she she was pleasured as much as he was. In short, he was just a better fuck than either my first friend or I was for that matter. She continues to look forward to his visits with enthusiasm.
I think you have something about her interest in multiple partners. Though it took me several years to convince her I'd be comfortable with her having sex with the first friend, once that hurdle was surmounted she didn't think twice about agreeing to ...... with the second. And, when I told her how hot I thought it would be if she had them both at the same time, she surprised me by saying, "Mike, don't you think I've already thought of that?"
That was not so easy to arrange, as both were married, and one lived out of state. However, logistics were overcome, adequate excuses were devised, and she ended up having two trysts with both of them at once. I was not present for the first (at her request). I was present for the second (at my request). She handled both of them easily, leaving them happy and exhausted after about two hours of wanton sex at each session. They returned her to me in a well-used condition which I found especially intoxicating.