Religious context in a nation has played an important role in determining how individual religions shape individual attitudes towards sexual relations. In Korea, 49% has no religion while 19% are Buddhists, 20% are Protestants, and 12% are Catholics in 2018. Using the 2008-2018 Korean General Social Survey (KGSS), we examined how the interaction of religious affiliation and strength of religious affiliation was associated with attitudes toward sexual permissiveness. While there is 25% point increase for permissiveness (not wrong at all/wrong only sometimes) for premarital sex relations (50% in 2008 to 75% in 2018) and 22% point increase for homosexual relations (15% in 2008 and 37% in 2018), there is almost no change for the permissiveness for extramarital sex (10% in 2008 and 11% in 2018). After adjusting for socio-demographic variables, we found that for both premarital sexual relations and homosexual relations, compared to weak Protestants, only those who are strong Protestants are less likely to have permissive attitudes and religious none are more likely to have permissive attitudes, but strong Buddhists, weak Buddhists, strong Catholics, weak Catholics are not statistically different. In terms of extramarital sex, strong and weak Buddhists, strong Protestants, strong and weak Catholics, and religious none are not statistically different from weak Protestants. Our findings may suggest that both religious affiliation and strength of religious affiliation are important factors in attitudes toward sexual permissiveness, and their effects differ by the types of sexual relations.