The problem is that a person's "lizard brain" is set to override rational thoughts. Although this effect can be decreased with training (something we do to all humans as they grow up) it's hardly an effective thing to rely on them following. Try telling a hungry person they can't eat without a napkin tucked under their collar (the nearest napkin store is 10 minutes away) or telling a person with a very full bladder they can't cut the queue for the toilet. It just isn't reliably going to work. They're probably going to ignore you.

(Although having sex isn't something that you need to survive (like eating), or something that will happen embarrassingly for physics reasons if you don't choose a voluntary place to do it (like peeing), it is controlled by similar mechanisms thanks to evolution being evolution.)