I don't know if a yaoi like this exists but I've read a lot love triangles and I wonder what they'll be like in a setting where it's acceptable to have multiple lovers. Sometimes it just seems like an easier solution lol.
Hm................. Thirsty for Love? Well, not exactly in a acceptable setting, but this manga is the only example I can think with "love triangle" been accepted by all the people envolved. You could try fantasies, like a prince with a harem, but I really don't know any other than Harem de Hitori.