We examined the relationship between moral worldview and number of sexual partners across 6 years in a nationally representative sample of 2,202 emerging adults. Using negative binomial fixed-effects regression models to control for all time-invariant confounders, we found that while changes toward more orthodox moral worldviews account for differences in number of partners during adolescence, this effect disappears as teens enter emerging adulthood. We explored two possible explanations for this change in effect, including (a) whether moral worldviews are simply standing in for external factors that are actually driving observed behavior and (b) how the effects of moral worldviews may depend on overall levels of orthodoxy. The results lead us to conclude that moral worldviews influence sexual behaviors over time; however, moral worldviews appear to have a significant protective effect only for individuals indicating a high degree of moral orthodoxy. These findings support the idea that moral worldviews impact health behavior during key developmental years.