Personally, I think
Do whatever the f*ck you want, as long as it isn't pedophilia, rape, and preferably with the knowledge of the spouse. It's not my job to dictate other people's sex lives, as long as they aren't causing harm
However, I do think that some people do it just because they are sex-starved/sex addicts, believing that they don't have some kind of problem, and some people let themselves be bullied into allowing a polyamorous relationship with their partner even if they don't want it