Here is some information I located regarding intermarriage:
The Mishnah says [Sanhedrin 81b] that the punishment for a Jewish man who has relations with a gentile woman is that zealots can kill the man. Rambam adds [Hilchos Malachim 9:7] that any gentile woman who purposely has relations with a Jewish man should be put to death. In addition to the punishment that a zealot can do to a man who has relations with a non-Jewess, a man who has relations with such a woman is subject to flogging from Beis Din because there are four different prohibitions upon such a woman. If the conditions are not able to meet the criteria needed for such a sinner to be flogged, the sinner would be punished by HaShem Himself with kares as it says [Malachi 2:11] [one who worships] .
The prohibition of intermarriage is so stringent that the Gemora equates intermarriage with idolatry [Sanhedrin 82a]