So-let me be frank here.
I fell in love with my husband and married him and I don't regret it.
A few years of hell with his ex literally tormenting me to the point of insanity (long story not going to hijack your thread) and I was a fraction of hte person I was when we got married.
I did still love him-but I couldn't even think clearly. Literally was unable to think through simple basic concepts without getting lost. All of this was purely a stress/emotional reaction to the ongoing trauma of dealing with his ex-wife and their ongoing court battle (in the end it lasted damn near to 8-9 years).
I had an affair with my best friend against my best friend's better judgement. He (C) attempted to talk me out of it-but the reality is he did love me and I wasn't ok and when push came to shove he was unwilling to prioritize Maca's feelings/needs over mine when I was so obviously the one in need at that time. Not that Maca's feelings didn't matter-they did. But he was fully responsible for why he had to deal with his ex and her issues-I was not and I was the one getting burned, not him.
So anyway-every person on here can agree that cheating is against every rule in the monogamy book and every rule in the polyamory book as well. But the reality is that sometimes even good people do the wrong thing. I was no exception.
I fell in love with C. Over the years I tried to walk away from that-but that ended up being realistically impossible. I moved away from both of them for several months. I cut off contact with C another time for several months. Maca moved out and we were seperated for nearly a year. All sorts of efforts.
But the reality is that I love Maca and I love C. Now here we are after much heartache, a baby of C & I's aborted (because it was during the affair and I couldn't bring myself to destroy the rest of the kids even though I thought abortion was wrong), nearly losing Maca's sanity, nearly losing mine and C struggling through to support the kids while we found what WOULD work.
I made a friend online who explained the term poly and asked if that might be part of my issue-that I was poly and trying to pretend not to be. I searched through every poly site I could find and in tears figured out it was totally "my problem". I found this site, started posting, started marriage/communication counseling and wrote a letter to Maca explaining to him all inside of a few days.
So I do get the idea of what you are saying.
Maybe "Mark" is a good guy who pushed himself into a mess. I don't know.
BUT-if he is there are some steps he MUST take asap to fix it.
1. is realizing if he is Mono-then he needs to DROP one of you RIGHT NOW. Pretending to be something you are not does psychological damage to yourself, every minute is more damage.
2. is accepting that your feelings/needs are no less important then his or Cathy's. No exceptions.
3. Identifying what it is he needs if he is staying with you in order to deal with his stressors (for example, I had to get a restraining order against maca's ex and insist that she may not call our home EVER or in any way have contact with me or my children EVER)
4. Make a commitment to being true to himself and uphold it.
5. Make sure that he is honest and open about life with whoever his partner is.
YOU need to insist that if these things can't happen NOW-then for your own well-being AND HIS-and because you DO LOVE HIM that you need to leave because it's not good for him anymore then you to keep playing this charade.
JMHO based on personal experience
"Love As Thou Wilt"