I am glad you are seeking counseling as you heal from the fundie brainwashing! That, to me, is the crux of the matter. As you well know now, that kind of christianity does lead to a lot of hypocrisy and cognitive dissonance. Parroting the fundie line of Christ and love and monogamy, and then, as you did, cheating and lying on the side. Til you break down from your soul being rotted.
Read francescasc's blog here, she's going thru it from the other end. Her h was the lying cheater, he said he was still Christian after she left the church, turns out he was paying for whores and strippers the whole time.
I know lots of poly people start out with a cheating scenario and manage somehow to salvage things and make it work with the partner they cheated with, as well as the person they cheated on. It can take years to heal and renegotiate, and sometimes (as in Loving Radiance's case) there is lingering resentment from the spouse despite years of actions taken by the cheater to rebuild trust.
So, not saying you can't salvage things with your Beth, by coming clean 100% with her new husband, but are you willing to put in the work? It's lovely to be loved and needed, but how much are you willing to go through to come out as her partner and be accepted by Mark and their families?