As positive psychological research burgeons, sexuality remains curiously absent from this growing discourse, and thus where to fit sexuality into models of happiness remains unclear, rendering such models, often, either incomplete or not fully applied. At the same time, research on sexuality and happiness has also begun to grow. But without a clear, agreed upon definition for what constitutes a good sex life, much of the current literature on sexuality and happiness lacks a theoretical basis. Seeking to redress each of these problems, this project introduces a proposed model of what constitutes a good sex life, the Thriving Sex Lives Model (TSLM). The project also introduces a proposed scale that is based on the TSLM, the Thriving Sex Lives Inventory (TSLI). As an artifact, the project includes a TSLI calculator (separate document). In Part I, the project details prior conceptions that approximate thriving sex lives. It then follows with an explanation of the theoretical basis behind the TSLM and TSLI. In Part II, the project includes a detailed explanation of each domain of the TSLM, which parallels the five domains of the validated PERMA model developed by Martin Seligman. Part II concludes with an explanation of how to use the TSLI calculator, and how to interpret a TSLI score, with implications for research and practice. The conclusion of this project highlights the need for research validation of the TSLI and more research on thriving sex lives, in general. The TSLI and an outline of the TSLM are included in the appendices, and a TSLI calculator is included in a separate document. This model and scale may help contribute to a budding effort at uncovering exactly what it is that makes sex worth having, and how sexuality can best contribute to a life that is most worth living.