It has been an issue a long time ago until now about the interfaith marriage. there was a time that not only the Jews but also other nationalities who usually do not marry people, not from their country. That is because there are many things to be considered. The culture, where to live ad many other things. But one of the biggest issues is the religion. If the couple does not have the same religion then they could not marry. It is not limited to the Jews but also in other places.
There is a religious group who do not marry people not belonging to their religion. The future husband or wife should be converted so they could marry. This was the case also in Israel that people who are Jews should not marry one that is not Jew. One of their basis is their belief in the old teaching of the bible not to marry gentiles. The Israelites are the chosen people and God had given them a command not to marry non-Israelites. They practice it until this time.
But it is already a reality that other Jews do not follow it strictly and so they had married non-jews but it is still a strong practice as it involves many things. That is why even if single jews said they can date a non-jew but when it comes to marriage it is another story as they would have to follow their standards and rules according to their faith and culture.