coming out of the stall

I do not understand why solicitation for unpaid sex occuring in a public lavatory is a crime. I could understand why committing a sex act in public is a crime. My concern is what makes a public lavatory different from the normal hetero or homosexual pick up bar where solicitation for sex occurs regularly and nobody is ever charged with a crime. Maybe it is because the ambience in a public bathroom is so conducive to sex, ugh.