It sounds like she LOVES feeling like a slut, at least in her fantasies, so it's a matter of taking the next step.
Just ask her next time what's holding her back? What is she afraid for. Listen to her and try to make sure to take those fears away.
She might be scared you will then demand to fuck other women, or that you will stop loving her or leaving her.
What you could do is to find a tantric masseur for her and bring him over to your house or book an hotel. You can talk to the guy to take it easy and see if and how she enjoys being touched by another guy while laying naked on the massage table.
Another option to ease into it is to invite one or two guys to come to your hotel room to watch you both in action and jerk off.
You can leave it at that for the first time or who knows, there might be some touching and fingering involved if she's up for it 
