I think it lurks beneath the surface and is a way in which she can free herself of guilt to explore extreme/kinky types of sex.It is an opposite for her as she is normally in control and seen as being a "good girl" and "prim and proper" in all respects but she has a very kinky side that needs an outlet.
I don't really know when that started but as long as we were giving in to our fantasies I found, more and more, that I liked the thought of her being used by other guys.
I even admitted to her that the most incredible thing that I could think of was to watch her sucking another guy's cock, especially if he had a huge cock and I were fucking her at the time.
She loves when I make her crawl to me, likes me to rub my cock all over her face, to be mouth fucked, have her hair pulled, to be cummed on, have her ass fucked (although this is infrequent and relatively new), light bondage, very rough sex, you name it.
While we were exploring these themes to make her happy, a little fantasy of my own began, her being with other guys.
This evolved into me realizing that I'd like to watch her as two guys used her at the same time.