It is interesting how things have changed for me/us in thirteen years. Now that there are five of us adults, three women, two men, and our children living together as a family, many people know. We don't discuss our sex lives with others any more than a vanilla couple would, but with us women all having children with both guys, it's easy to figure out.
BTW, my sister didn't remain monogamous for very long - she is a serial adulteress. Her husband finds out, they fight, split up, and make up. It's become a predictable pattern.