These properties, after all, are not only moral but isn’t ethical, it’s betrayal.Betrayal, secret affairs and so on, defeats the point of being nonmonogamous.
The point, though, is that there is nothing significantly different about being sexually active with other people as well as a primary partner; since, like any relationship, what we want from these might not end up happening.
That is not a reason to err on the side of absolute avoidance, however.
This is clear from the way people react to homosexuality, sex work, antinatalism (not having children), paedophilia, pornography, incest, and so on.
All these topics are often discussed with knee-jerk reactions from all quarters - not just conservative religious people.
In this case, the only way to satisfy your need is to be secretive about it.