“Frei” is the Yiddish word meaning “secular” and is a quite proper way to refer to non-Orthodox Jews, and was used by themselves back when there were secular Jews speaking Yiddish as a first language. It is not a perjorative. The literal Hebrew translation, “Hofshi” is used in polite company, and is even in the zionist/Israeli anthem.
“Shiksa”, which is actually a Yiddish version of the Hebrew word for a “worm” (an “Av Tumah”) has strong sexual connotations, as does the masculine equivalent (“Shaggetz”) , and probably should not be used in polite company. Its widespread use is because the Yiddish spoken by most pre-war (meaning, before the holocaust refugees arrived and hareidi Jews became common in the USA), was not “polite”. YWN probably should block posting with the word. The correct way for a Ben or Bas Torah to refer to a non-Jews in Yiddish or Hebrew is as a “Goyah” or a “Nachrit” or an “Annenah Yehudit”. If you need to refer to a female worm, one should use the correct Hebrew term pronounced as it is in Lashon Kodesh.