Crazy there's so many serious comments here. Geuss I'm adding to it tho. I am actually in a polyamorous relationship, and it's way simpler than anyone is thinking. No part of it being polyamorous means that the same rules don't apply. If my partner has sex with someone who isn't one of their partners, or found another partner without keeping me involved then they would still be being unfaithful. If you are going to be in an OPEN relationship, you need to be OPEN to your partner about said relationships. End of story.
Crazy there's so many serious comments here. Geuss I'm adding to it tho. I am actually in a polyamorous relationship, and it's way simpler than anyone is thinking. No part of it being polyamorous means that the same rules don't apply. If my partner has sex with someone who isn't one of their partners, or found another partner without keeping me involved then they would still be being unfaithful. If you are going to be in an OPEN relationship, you need to be OPEN to your partner about said relationships. End of story.