When black widow spiders mate, the female always kills and eats the male.
To understand the facts about black widow mating, you must first understand that there are many different species worldwide in the black-widow group (the genus Latrodectus), and three different black widow species in the United States alone, two in the east and one in the west. These species do not all behave alike. Moreover, in the past most observations of mating took place in laboratory cages, where males could not escape. The only known Latrodectus species in which mate cannibalism in nature is the rule, not the exception, are in the Southern Hemisphere. Of U.S. species, mate cannibalism occurs sometimes in Latrodectus mactans, the eastern black widow, but most males survive to mate another day. In the other two black species, including the western black widow L. hesperus (only species west of Kansas), mate cannibalism has never been observed in the wild!