I don't understand Pol/ENM. I honestly believe it's a label, which today, if anything is labelled then it's validated, and therefore above reproach. What's it really about? Not emotional love because that's why you choose a primary partner. It's an excuse to behave without responsibility and a "proper" way to avoid making excuses.

If you want sex with other people and your partner does as well, then add a player to the bedroom from time to time. That's cool, good times, and obviously--no secrets.

It doesn't make sense. If you're allowed to be emotionally and sexually involved with others outside your primary relationship, then it's naive to think you won't be/feel betrayed by one side or the other.

While I always aim to be supportive of my kid, I would be blunt and honest in this case and say, "Yeah, that's not going to be the paradise you think it is, and you'll ending getting hurt by multiple people at once instead of just of one. Decide what you want--one person to love and trust and play with, or run around with whomever at your fancy?"