I can answer from just the poly side of it. If you are in a closed relationship that allows threesomes you set ground rules. Generally neither partner should be sleeping with a third solo unless it was agreed upon beforehand. It is pretty taboo to do something like that. I have known couples that were semi closed who would allow it with agreed upon thirds but that was pretty rare.All true. But once Lena "outs" herself with Louise and (sort of) Ivy, would it be okay for her to fuck Louise or Ivy solo after their threesome with Ian?. And if so, wouldn't it also be okay for Ian to fuck them solo too?. I get the "I fuck my hot friend and then bring her to our bed" but I'm not sure what's the etiquette after that, never been in that situation myself. Also, just out of curiosity, any woman or bi guy available to offer some perspective on how it would work if Ian fucks Jeremy and offers Lena a threesome afterwards?.
It is 100% cheating if a partner sleeps with a new person and says we should have a threesome with them unless that privilege was given beforehand. What if your partner was not interested in a threesome with them then you 100% just cheated on them. Generally if one partner is allowed to just sleep with whoever and then say hey we should do a threesome you are in a semi closed relationship.