Friendships may involve some gauge of transitivity. In other words, a person may become a friend of an existing friend's friend. However, if two people have a sexual relationship with the same person, they may become competitors rather than friends. Accordingly, sexual bag with the sexual partner of a chum may damage the friendship (see allegiance triangle). Sexual relations between two friends tend to alter that relationship, either by "taking it to the next level" or by severing it. Sexual partners may also be classified as friends and Interracial Dating the sexual relationship may either enhance or depreciate the friendship.

Though nuanced, there is a significant difference between girlfriend and boyfriend, and girl alter ego and boy friend. In a strictly grammatical sense, a girlfriend or boyfriend is an 'individual of significance' with whom precise shares a relationship. A girl classmate or boy friend, however, is ingenuously a bedfellow identified on the basis of gender. Since the pronunciation is the same, these words may occur to be false friends.