I am in respectful disagreement with what most most here on the 'Board say about your proposal. My personal experience is that I was very jealous of the thought that my boyfriend (who later became my husband) would have sex with another woman despite he allowing me to continue having sex with my ex-fiance (to this day) and others. Eventually, I did indeed "stare the demon in the face" and arranged for one of my girlfriends who thought hubby attractive to have sex with him. It continued for a while and was as I've described elsewhere like the burn one feels after a hard run, or getting off a scary roller coaster and wanting to ride again. I became hooked on the very thing I dreaded. It got amped up even more as I found women who would let me watch, then participate. It also brought out my Lesbian side, which I now enjoy as much as hetero sex.
As for how to start - the bit about her sex life before you is the gentle path to take. Ask her about her youth in a nonsexual way, pleasant memories of where she went, who she was with, what she did. Some of them will be past lovers and sex partners. Let her talk, do not judge or press her, and eventually after months or years she will open up to you about who she had sex with, what they did, what she enjoyed. Make it a cause for celebration and an opportunity to open up sex between you - what you do together, her fantasies. From there it is a short step to talking about her desires for real people, atheletes, movie stars; then people you know, friends, neighbors. Let it start the way my husband did, letting her do what she wants first. Make her happy, then possibly she will share as well. Good luck on your journey.