It outweighs any synagogue or temple, even the Holy Temple built by King Solomon. By marrying a non-Jew one thereby ends over 3,000 years of Jewish continuity, effectively cutting oneself and one's offspring off from what it means to be Jewish. There have been many other arguments offered against intermarriage, below is a summary of some of the most famous. Today there are only 13 million Jews in the world. The answer is that the silent holocaust of assimilation has caused them to disappear as Jews.
Intermarriages are twice as likely to end in divorce as same-faith marriages.
Some reasons for this are the different identities of the spouses and the differences in culture and family. A gentile who converts in superficial and insincere conversion only for the sake of marriage does not create a new identity that is now Jewish. One is granting a victory to anti-Semites who seek to destroy the Jewish people. Think of what has been sacrificed in the past, by our own ancestors to keep their Judaism. And think of the heritage that is being sacrificed for the sake of personal reasons.