I think they exclusively cater to that audience. People that are married and are seeking a relationship.
The point still somewhat stands that if you and your partner had open-marriage or something, your friends and family might find out.
In which case, just pretend you were cheating on her and carry on (if price of outing you as a non-monogamous couple is too high).
So, even ignoring that high horse you're on: in your not so humble opinion, are all registrants on ALM guilty of marital infidelity? That's assuming guilt by association, and flies directly in the face of your own constitution.