Tosfos [Yevamos 25b] says that a person who said that his conversion was invalid is believed to forbid himself to a Jewish girl.
What is the איסור for a Goy to be with a Jewish girl?? She is forbidden to him but not him to her??! And as far as she is concerned - we don't believe him! For him we say "שויא אנפשיה חתיכה דאיסורא" but that doesn't impact her!
Asking for a Gentile man who likes a Jewish girl רח"ל.
