The practice of sexual cannibalism, primarily characterized in several species of insects and arachnids, is frequently misunderstood.
The Mantis religiosa species of mantis requires that the head of the male be removed for proper ejaculation to occur. In most of the other 200+ species of mantis, this behavior is very rare. In addition, cannibalistic behavior in black widow spiders is also quite rare in the wild.
It is thought that most early observations of male/female behavior occurred in laboratory conditions in harsh lighting, a jar, fishtank or other confined space following stressful handling by the observer. In several of these species, the female is much larger than the male, and under stressful conditions, is very likely to attack him.
When this behavior does occur in the wild, there are several variations:
Pre-copulatory cannibalism may occur as a mechanism for female choice. For example, several males will approach a fertile Orb Weaver spider in its web. (image above) The female may eat those males she does not find suitable mates to prevent them from mating with her.
Given that this behavior occurs in predacious species, pre-copulatory cannibalism can sometimes occur due to "mistaken identity".
In the Yemenese species of spider, Tidaren argo, the "palp" or male reproductive organ, is torn off by the female following insertion. She then consumes the male, leaving the palp in place which functions independently to continue fertilization for up to four hours.
In Ceratopogonid midges, the female eats the male during mating. These short-lived insects generally only get one opportunity to mate, and the female consumes the male as nutrition, as well as the extended fertilization that occurs after his death.
In Australian redback spiders, the male performs a "sexual suicide" by placing his abdomen in the mouth of the female during mating. This appears to prolong the copulation. In addition, a female that has eaten a male will not copulate with another male spider, ensuring the reproductive success of the sacrificed male.

In humans, there is no anthropological evidence of sexual cannibalism in even the most primitive of rituals, this behavior is reserved for those with psychosexual disorders. For examples of sexual cannibalism from these ugliest of animals: see CourtTV's crimelibrary.com