There is absolutely no excuse for deceit - ever. That said, I find your expectation that your husband just magically stop having feelings for the woman you gave permission for him to pursue unrealistic at best and cruel at worst.
I say that because I have been in that woman's shoes. The wife encouraged the husband into a poly relationship with me, waited just long enough for us to grow to love each other, then went 180 degrees the other way. She also blamed her abandonment issues. While I don't want to minimize her past trauma - or yours - the trauma she inflicted on the two of us by setting us up to love one another, then ripping us apart was almost more than I could bear.
I must say, unlike your husband, we did not cheat, nor deceive her. We respected her wishes that we disengage sexually, but neither us knew how to "un-love" the other. Unfortunately, sexual disengagement was not enough. She could not tolerate that he had any feelings for me whatsoever. She eventually demanded that we not see each other all. And so we cut contact with one another.
I had been out of their lives for a year when they separated, and are now divorcing.