90% of all romance movies/books/tv shows.
One of the most common themes I've seen in these stories is the love triangle as the central problem that the story rests on. In most cases, there's no solid reason a poly or poly-esq arrangement wouldn't work out aside from the characters being barely emotionally stable enough for one relationship, let alone two.
"You MUST choose! Spend your life with me, Fernando the baker, or with him, Constatine the butcher!"
Real mature, Fabio.
I am as direct as a T-Rex with 'roid rage and about as subtle. It isn't intended to cause upset, I just prefer to talk plain. There are plenty of other people here who do the nice, polite thing much better than I can. I'm what you'd call a "problem dinner guest."