The answer is because this happened by chance, in a time where she was hurting (b/c my divorce wasnt final and we're supposed to be getting married in 4 months) She doesn't have a lot of friends, she's introverted and because now that we've had this talk, she knows I'm not (at this point, at least) open to poly.
We've talked about this very point. I asked 'If this does fizzle and we can go back to being 'us', would this ever happen again?' She said no and we talked about the 'why'. I understand her answers and I believe her.
The odds of her falling again, for anyone, are about as good as ME falling for someone else. I Am completely ok with those odds