There are very strong warnings in the Book of Devarim for Jews to remain separate and distinct from other nations.
The commandment forbidding intermarriage is found at the end of ואתחנן. In certain instances, there is no tolerance for idol worshippers or those who incite Jews to turn away from their heritage.
Many find Judaism to be too harsh in its treatment of those who have gone astray. Rabbi Soloveitchik makes an amazing distinction between an individual and one’s ideology. The individual can do Teshuva especially if his sin is giving into lust or passion. But when one espouses ideas that threaten and undermine Judaism, there is little tolerance. The Rambam says that to be merciful to such individuals is really cruelty.
This is a very important distinction: the individual as opposed to an ideology.