Consistent with symbolic interactionism and motivation research, the study explored the meanings of sexual behavior in romantic relationships in a sample of 3,003 online respondents. Starting with a pool of 104 respondent-generated items, Exploratory and Confirmatory Factor analyses in separate sample halves revealed a stable set of 9 dimensions within that item pool that formed 2 higher-order factors representing positive (to share pleasure, to bond, to de-stress, to energize the relationship, to learn more about each other) and negative (to manage conflict, as an incentive, to express anger, and to control partner) meanings of sexual behavior within relationships. Item Response Theory analyses helped select the 4–5 most effective items of each dimension for inclusion in the Meanings of Sexual Behavior Inventory (MoSBI). Generalizability analyses suggested that the MoSBI subscale scores continued to show high levels of internal consistency across a broad range of demographic subgroups (e.g., racial/ethnic groups, gay and lesbian respondents, and various levels of education). The MoSBI subscales demonstrated moderate and distinct patterns of association with a range of conceptual boundary scales (e.g., relationship and sexual satisfaction, emotional support, negative conflict behavior, and frequency of sexual behavior) suggesting that these scales represent novel relationship processes. Consistent with this, analyses in the 862 respondents completing a 2-month follow-up assessment suggested that the meanings of sexual behavior predicted residual change in relationship satisfaction, even after controlling for frequency of sexual behavior within the relationships. Implications are discussed.