Some people find pain pleasurable during sexual acts, but why? Experiencing pain from a knife cut in the kitchen or pain related to surgery.
Someone into masochism gets sexual pleasure from being hurt: they are turned on by pain.
Algolagnia is a sexual tendency which is defined by deriving sexual pleasure and stimulation from physical pain, often involving an erogenous zone. Studies.
S & M, they are beginning to understand, offers a release of sexual and emotional energy that some people cannot get from traditional sex.