Sex with Chickens in Pink Flamingos (John Waters, 1972)
Bestiality scenes can be quite controversial (see the cat rape in Leolo, for example), but animal rights groups have less to protest with a film that merely simulates animal cruelty than a film like Pink Flamingos, which definitely appears to be exploiting and harming a live chicken during an infamous moment of lovemaking between two characters who crush the fowl between them. It may not be the most disturbing moment of the film, but it is one of the more contentious.
Long before Brokeback Mountain and Milk were protested for their tasteful depictions of homosexual lovemaking (Christian groups predictably slammed them for being gay propaganda), Cruising was controversial for depicting gay sex as fetishistic, depraved and violent (gay groups slammed it for being anti-gay). The film originally opened with a disclaimer stating that it portrays only “one segment” of the “homosexual world” and is “not meant to be representative of the whole,” but it didn’t help matters. The film was still considered homophobic, failed at the box office, and worst of all, led to copycat crimes against gays.