In sexually monogamous relationships (SMR), sociosexuality, or the predisposition to engage in extradyadic casual sex, is negatively associated with relationship outcomes. However, mutually consenting to sexually non-MR (SNMR) does not hinder relationship outcomes. Recent research has extended these findings to the phenomenon of online extradyadic sexual behavior. The aim of this study was to examine whether this sexual agreement moderates the association between sociosexuality and relationship satisfaction in a sample of romantically involved heterosexuals registered on a dating Web site directed at other romantically involved heterosexuals—Second Love. A sample of 329 Portuguese heterosexuals (66% men) aged 18–68 (M = 41.47, SD = 10.19) completed a web survey. A bootstrapped moderation analysis showed the expected negative association between sociosexuality and satisfaction, b = -0.18, p = 0.013. However, this was moderated by sexual agreement, b = 1.12, p < 0.001. While individuals in SMR showed the negative association, b women = -0.46, p < 0.001, b men = -0.67, p < 0.001, for those in SNMR the association between unrestricted sociosexuality and satisfaction was positive, b women = 0.63, p = 0.002, b men = 0.46, p = 0.006. Gender did not moderate these results. The current study suggests that dating Web sites for romantically involved heterosexuals are not necessarily negative for relationship outcomes as long as there is a consensual agreement for sexual nonmonogamy. This adds to the growing body of literature examining online sexual behavior and sexual infidelity. Also, implications for sexual health and risky sexual behaviours are suggested.