It is totally forbidden for a Jew to marry a non-Jew. There are no concessions or exceptions to this rule.
There is, of course, no Kesuvah. The obligation of kesuva is a Rabbinic enactment to protect the wife's rights in the marriage and to ensure that divorce is a last resort. This obligation applies only to a marriage the Torah recognizes, not one which ...
Reading the Book of Mishlei, I surprisingly stumbled upon an apparent answer to my question.
In Mishlei 9:7 we read:
He who chastens a scorner takes disgrace for himself, and he who
reproves a wicked man, that is his blemish.
and he who reproves a wicked man, that is his blemish: It is a blemish to the one who reproves [him], for ...