Not sure how being the secret other woman can be viewed as a "good thing going" for both parties. For him it's great. He gets to have his full time relationship with his wife....doesn't have to feel the guilt and shame of dumping her once she got sick. Maybe the nurse/caregiver role helped him and his wife with any guilt he might of had in starting such relationships. In the beginning it was biological needs being satisfied. He got 75-90% of his relationship needs met from wife and got the 10-25% physical/sex piece from others. Now that love has entered the picture all those numbers have shifted and the lines have gotten blurred a little but there's still a split.
Conversely, newtoday, has once a week face time interactions and limited intra week contact....and other women secret status with friends and relatives. Seems unbalanced to me. What's the % missing for her 40-70%??? 10-15%???
Everyone in this is really mono and is participating in poly by forced circumstance.