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OY. What stupidity. Although the word "gay" was not widely recognized as meaning "homosexual" until the mid-20th century, according to Wikipedia, "It was originally used to refer to feelings of being "carefree", "happy", or "bright and showy"; it had also come to acquire some connotations of "immorality" as early as 1637.

Someone should explain to that teacher that the meanings are closely related. Gay = light and carefree free/unconstrained sexuality sexuality outside the bounds of acceptability homosexuality.