It's gross to masturbate in a public location.
Bob is aroused by actress that he sees when he in the movie theater.
Bob wants to blow off some steam to clam the urge.
Bob resists the urge, but rubs one out at home after the movie is over.
Bob pulls his penis out while sitting in the theater and starts masturbating.
Bob feels better and know one else knows what Bob has done.
The cops are called and Bob is arrested for indecent exposure.