The one who didn't get away. A male Nephilengys malabarensis
snapped off his genitals (red box) in the female, but it was eaten anyway.
I suppose it's better to snap off your genitals rather than be devoured by your partner after mating, but it's not that much better. Here's a solution that the male orb-weaver spider Nephilengys malabarensis developed to increase its chance of survival after mating with a cannibalistic female:
Daiqin Li at the National University of Singapore and his colleagues studied the species and found that after the male breaks away his severed organ continues to pump sperm into the female. This allows him to fertilize her remotely, while denying entry to other males. Even though the male cannot regrow his genitals and so renders himself sterile, he increases the odds that he will father the offspring of his one and only mate. [...]
Li thinks that this bizarre strategy, found in only two spider families so far, evolved to counter the female’s penchant for cannibalism. “The females are very aggressive and 75% of them kill the males during sex,” he explains. “The duration of copulation is also very short, and the females initiate the break-off.”
Previously on Neatorama: 30 Strangest Animal Mating Habits
So if the genital can't re-grow it will render the male's continued existence irrelevant.
I think it's prime reason is to prevent other males from copulation.
This is a testable hypothesis.