Let's remember that MOM can mean mixed "open" marriage or mixed "orientation" marriage. Just because a person has a SSA and is honest about having a different orientation doesn't mean they will require sexual intercourse outside the marriage. I think it would be a great challenge, but there are some people who are up to the task.
Hopefully mk33's husband is one of those people. If he can admit to a SSA but remain in love with her and monogamous to her, then it can work.
Heck.. I had a MOM for 16 years... I just didn't know it until the end. It worked reasonably well for 11 years, faltered a bit for 4 years and then turned into a dumpster fire for the last year.
It all depends on the spouse with the SSA and how strong their convictions are. It also depends on how long the str8 spouse can be happy living with a lack of trust and worry over the situation.
-Formerly "Lostdad" - I now embrace the username "phoenix" because my former life ended in flames, but my new life will be spectacular.