[user104658]

Anyway, my answer is that life is life (and you only get one!) and adults are adults and everyone is different, so what it comes down to at the end of the day, is honesty. Someone who would lie / cheat / betray is not someone you should easily trust, and I would say likewise, someone who is OK with being "the other person" knowing that someone is being betrayed, is probably also not a good person. The concept of ethical non-monogamy though is not really a moral question so much as a personal choice; if everyone involved is OK, it shouldn't be for anyone else to judge. I do often feel like people coast through life making fewer meaningful connections than they could (obviously that doesn't have to involve sex! Just the whole thing, close friendships, emotional connections) and end up in their elderly years with some regrets? It's definitely not black and white, for me.