It feels to me like you're boxing yourself in again. You seem to regret not being able to date/spend time with her the way you could have over the previous months. Why turn down the chance now that you know it's there?
Yes, it sucks that the bf isn't automatically ok with you and she being together sexually (especially since you have been already, but that's just my pov) and might never be ok with it. But sex isn't everything. Since I seriously doubt sex is all you want from her, why not go with the romantic feelings you're both having? Do you really think not being with her at all is better than only being able to be with her in certain ways?
Yes, your feelings might get even stronger, and yes, it might hurt even more later, but as you said, your feelings are already there. You're absolutely right that you can't put them back in the box so it's already going to hurt if you try to cut yourself off from her now and try to transition back to just friends. Why not have some joy with her in the meantime? You never know what the future will bring or what will happen with her and the bf. To go with a cliche: life is short. As long as it brings you overall more happiness than not, I would say take love when/how it's offered.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack