In 1980, a Brazilian college student had a yechidus (private audience) with the Lubavitcher Rebbe.
During the Yechidus the Brazilian student brought up his relationship with a non-Jewish woman, wanting to hear the Rebbe's opinion.
"Rebbe, my girl friend, who is not Jewish, and I, are thinking of getting married. What would the Rebbe say about that?"
"There are," the Rebbe replied, "many aspects of our lives over which we have no control. Many physical conditions, as it has been scientifically shown, cannot be altered since they are a consequence of our genetic makeup, which has been inherited from past generations. There is not much, generally, which can be done by others to help these conditions.
"However, our daily functioning is primarily influenced by decisions we make throughout our lives. When people make dangerous decisions, we expect those around them to work to prevent the danger. If, for example, we hear someone planning to commit suicide, even if they say that they clearly know what they are doing and have made a conscious decision to proceed with the suicide, it is universally assumed that we will do all we can to stop that from happening.
"Our spiritual lives are shaped by the choices we make. In a sense, the results can be more tragic than suicide. Unlike suicide, which occurs momentarily and no longer distresses the perpetrator, a dangerous decision about one's spiritual life will hassle a person for many years. So, we must do all we can to dissuade a fellow Jew from marrying a non-Jew.
"May G-d bless both you and your girlfriend to find the right person for yourselves, and then, with your respective spouses, you will both live happily. Meanwhile, you should discontinue any relationship with her, and it should never be renewed. You should go from strength to strength."