The sexual system evolved to motivate reproductive acts. As such, its manifestations (e.g., sexual desire and behavior) are not necessarily related to emotional attachments. Still, within romantic relationships, sexual desire can motivate the attachment bonding process and lead to intensely meaningful experiences that affect the quality of a relationship and its fate. In this article, we review published evidence indicating that sex promotes enduring bonds between sexual partners. We then introduce a relationship stage model that delineates the functional significance of sexual desire in relationship development. This model suggests that although sexual desire influences the initiation, development, and maintenance of attachment bonds, the contribution that it makes varies over the course of relationship development and across individuals and circumstances.