Lama Ole’s answer:
I would say that if there isn’t any jealousy, it can work very well with more partners. In Tibet, for instance, it was more likely that a woman had more than one man and not the other way around. Nevertheless, in most cases, a one-to-one relationship as the core family with a certain degree of openness is the best solution—though there are also cases where it can work well with several partners.
But one always has to take care that nobody suffers or is taken advantage of. Every time people come together, it should be based on the idea of helping another being to grow and to develop.