Just a little update!
I wanted to say thank you to everyone who has helped me with this. You all seriously got me thinking and because of that, I'm in a much better place!
I've basically put my foot down. I've talked to her, shown her this thread and shown her that she is kind of acting like a little kid in a candy store - she thinks she's being supportive; but really all she's doing is making her own stress worse. And ours.
Seems she got burnt out on what she thought was nurturing, because her "single gal" behaviour was causing us too much stress.
She's starting to see that she could have avoided all this in the first place with one simple, considerate, talk before she started it all!
She's ready to sleep with a new person and actually *discussed* this with me rather than dictated it to me. She's also starting to listen to me and not treat me like a student of her poly knowledge. After being on this forum, my knowledge is actually greater, haha
So thank you, again, everyone! If she becomes more considerate and wonderful, I will be a happy girl. If she doesn't - I now know how to stand my ground.