The correct answer is endogamy.
Endogamy is the practice of marrying within a specific social, ethnic or religious group or cast. Anybody from outside such group is being considered unsuitable for marriage and rejected.
Endogamy is popular among many religious groups and common in Judaism. This religion usually mandates endogmany but allows marriage for converts. Orhodox Judaism is firm on keeping the traditional requirements, but many other Jewish religious movements are more permissive with interfaith marriage.