I read somewhere, I think it was on this forum, that it is not uncommon for triads to ultimately become a V relationship.
While my girlfriend is my secondary, it is important to understand that the love I have for her isn't. Before me, she had dated other married guys, and had on occasion been left sitting at a restaurant waiting for her date to show up. She has been stood up simply because the guy's wife made lasagna that particular night.
While the dynamics of a relationship may be different and unequal, we should always treat people as equals.