Recent research has revealed some inconsistencies in the traditionally negative attitudes towards consensual nonmonogamy (CNM; sexually and/or emotionally nonexclusive romantic relationships), with some adults reporting fairly neutral attitudes. These inconsistencies may be related to the effects of socially desirable responding when adopting self-report (e.g. explicit) measures. Thus, the current study assessed young men and women’s implicit attitudes towards CNM (using the Implicit Association Test) in order to bypass issues associated with social desirability bias. The results from 204 college students (81 men, 123 women) revealed that, despite reporting neutral explicit attitudes towards CNM, young men and women demonstrated a strong automatic preference for monogamy (mean D score = 0.71; SD = 0.32). Furthermore, the relationship between explicit and implicit attitudes was clarified by assessing the extent to which participants were likely to engage in socially desirable responding. Implicit and explicit attitudes towards CNM were more closely related among those less likely to evidence social desirability bias as compared to those who were more likely to fall prey to this bias. These findings highlight the importance of assessing implicit attitudes and provide evidence of the strong social stigma surrounding CNM.