Friendships are not monogamous by necessity. Two people in a friendship don't need to exclude other people from their relationship. A friendship can best be thought of as two people side by side looking forward toward a common goal. It's an odd form of love in which people develop a relationship without relationship as a goal. Scientific achievements have come out of tight-knit friendships, as have works of literary genius, as well as victories in wars (due to the tight camaraderie and mutual trust of soldiers).
Our earliest friendships are coed, then imprecisely homoerotic, as we reach the age at which tribal peoples form cadres of hunter-warriors to protect and feed the clan, then homophobic for the sake of family life, and at last relaxed and coed again.
We often mistake infatuation, passion or limerence for love, but real love cannot exist without a firm basis in friendship. Popular culture has done us a great disservice in our understanding of romantic love. From a young age, we watch movies and read books that form the scripts of our adult relationships. But popular culture usually gets it wrong, often in the name of entertainment, and ends up confusing love with limerence, which is precisely what most of us do.
According to psychologist Robert Sternberg, who has invested some $20 million in research grants studying the subjects of love and hate, there are actually three stages to love: Passionate or romantic love (limerence), Companionate love, and finally, Committed love. Without the progression to the last two stages, a relationship based on infatuation, passion or limerence is subject to disappear very suddenly. And the last two stages must include an intimate friendship.