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 (Watson and Crick), as have works of literary genius (J.R.R. Tolkien and C.S. Lewis), as well as victories in wars (due to the tight camaraderie and mutual trust of soldiers).