19. This sounds like silly, insincere "advice" in a list of useful, sincere advice. Does the author really mean this? If so, it's completely absurd, like unironically saying "never write an action scene without ninjas or zombies." I guess perhaps the intent was to say "Don't have unnecessary sex scenes just for the heck of it."
I think what he means by this rule is that by not allowing the characters to actually sleep together you keep tension in the story. Which, in a way, is saying you shouldn't throw in sex scenes for the heck of it. So nevermind.