I don't know about their specific arrangement, but in some relationships, having other partners is not only accepted, but actively used to reinforce the primary relationships.

If you accept your partner has those urges, you can either treat it antagonistically and have those urges become a problem in your relationship or you can absorb them into your relationship and have it be strengthened by them.

You either become the one being cheated on and weakened or the one in control with a relationship strong enough to not be threatened by some side action, which in turn makes you a rarer and more valuable mate.

Or as a less cynical take, there's also the phenomenom of compersion, the ability to feel joy from a partner's happiness with other people. If you truly care for somebody and want them to be as happy as possible, you might be willing to set your ego aside to create that experience. And of course, that sort of security is again very attractive and a rare trait in itself.