Sex on the beach is the right usage. This is because beach is open & the act is done on the beach . Sex in the bedroom is right usage. This is because bedroom is enclosed by walls & the act is done inside the bedroom. Sex on the bed in the bedroom is right usage. This is because bedroom is enclosed by walls & the act is done inside the bedroom AND the act is done on the bed & not inside the bed. ============================================================== Even if you're referring to the cocktail, sex on the beach is the correct usage. I don't think this has changed in English since long time