Oh, I actually feel rather sorry for this woman. :/ Here she may well think that you are pursuing her as a serious interest, and yet under the rules of your relationship, you and she can only be intimate once -- that's actually rather tragic in a way, if she's developing real feelings for you. I would tell her at once, via email, and let her decide if she wants to go any further. Also, I would strongly consider renegotiating your arrangement with your wife. It just seems cruel, to the other women if not to you. I mean, go to swinger's clubs if you're just going to sleep with a woman once and then never again, don't go through the motions of trying to develop something meaningful.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.