United Airlines has just been accused of allowing a male passenger to masturbate next to a 17-year-old girl on board one of its flights last year.

Monica Amestoy -- now 18 -- has filed a lawsuit against UA, claiming she was flying back to L.A. from a high school debate tournament in New York last October when the incident occurred.

According to Amestoy, represented by Gloria Allred, a male passenger seated across the aisle from her exposed his penis to her during the flight ... and masturbated.

Amestoy says she complained to a flight attendant multiple times -- and the flight attendant did nothing to stop the man from continuing his behavior ... she wasn't moved, and the passenger wasn't addressed in any way.

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