Sex can be defined fairly adequately in physiological terms as consisting of the building up of bodily tensions and their release. Eros, in contrast, is the experiencing of the personal intentions and meaning of the act. Whereas sex is a rhythm of stimulus and response, eros is a state of being. The pleasure of sex is described by Freud and others as the reduction of tension; in eros, on the contrary, we wish not to be released from the excitement but rather to hang on to it, to bask in it, and even to increase it. The end toward which sex points is gratification and relaxation, whereas eros is a desiring, longing, a forever reaching out, seeking to expand.

Ch. 3 : Eros in Conflict with Sex, p. 73 - Love and Will (1969)