Blissfully naïve 1977 Star Wars TV spot advertises "forbidden love" between Luke and Leia
The advertisement aired on TV before 1983's Return Of The Jedi revealed the two characters are siblings

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A man and a giant bear-ape consider their friendship with a horrible pervert. Screenshot: Voca Productions
Back before Game Of Thrones made horrifying incest plot lines a selling point, audiences hadn’t been primed to squint their eyes with nauseous suspicion at genre stories featuring two vaguely similar-looking lead characters in a romantic relationship. Because of this, the people who saw the Star Wars movies as they released in theaters had no reason to retch quietly when they watched scenes featuring stuff like Luke Skywalker, Jedi warrior, leaning back with smug gratification after smooching his sister—or to watch advertisements for A New Hope that promoted the movie’s “forbidden love” without tossing their cookies in the process.