Spider sex carries a very definite danger for male arachnids: genital loss. Spiders can actually lose their reproductive organs during intercourse, turning them into spider eunuchs. But, unlike humans, this "castration" actually turns the spiders into even more aggressive fighters.
It's all down to the unusual reproductive strategies of the spider species Nephilengys malabarensis. The males want to ensure that their female mate won't be impregnated by anyone else after they have completed copulation. Since actually killing the female would defeat the purpose of the whole exercise, the male does something only moderately less drastic. He breaks off his reproductive palps and leaves them inside the inseminated female.
This blocks off the female's reproductive opening, which would theoretically make it impossible for any further pregnancies to take place. But there's one small problem with this otherwise foolproof plan - rival males can simply remove the palps and have sex with the female anyway. To protect against that monumentally embarrassing end result, the newly created spider eunuchs become extremely aggressive, jealously guarding their mate and fighting off any other male that approaches.
Researchers Simona Kralj-Fišer of the Slovenian Academy of Arts and Matjaž Kuntner of the National Museum of Natural History in Washington, DC observed this behavior in N. malabarensis, but they also found another spider species that takes a slightly different approach. The Madagascar spider Nephilengys livida also featured castration in 70.83% of all of the species's sexual encounters, and most of these prevented further reproduction.
What's unusual is that the eunuchs of N. livida didn't become better fighters. In fact, they barely bothered to guard the females at all and were generally less aggressive than virgin spiders. The researchers have two possible explanations for this. One possibility is that these spiders have lower population densities, which means there's less competition and so a lot of male aggression becomes redundant. The second and rather more disgusting possibility is that N. livida males have evolved a more efficient system for "plugging" the female reproductive opening. You know, I'm starting to realize why I don't spend a lot of time thinking about arachnid sex.