While I get that honesty is much preferred to deception, I don't think you are pushing Beth to divulge the nature of your early relationship to Mark out of some sense of nobility; it sounds like you want to fuck up her relationship with Mark. That is NOT cool! How do you think she is going to feel if she loses Mark? Happy? Do you really think that would be good for your relationship? Do you think she is going to joyfully want to marry you after that?
It sounds as if you do not feel you can comfortably be poly AND participate in family / community events. You wouldn't be the first person to find this challenging. You wouldn't be the first person to decide he functions better in monogamy. I would say, your timing in regard to Beth sucks. And I would say efforts on your part to harm her relationship with Mark are terribly unkind. Unkind to Beth, and evn more so to Mark - who did his best to adapt to the situation you and Beth created behind his back.