What I do know is that it absolutely sucks to be in a relationship when you think your partner is in love with someone else.

Assuming you don't think being in love with multiple people is perfectly normal. And I really wish Prudence had acknowledged that this paradigm exists.

I hate the conflation of monogamy (which I think of as "agreeing to have a sexual/romantic relationship with only one other person) with "close friendships between people of an appropriate sex to each other are forbidden."