The behaviour is called traumatic insemination. The male's genitalia end in a hardened, curved paramere that punctures the female's cuticle. Sperm released into the haemocoel travel through the body fluid to reach the ovaries. Each mating is an injury, and repeated matings shorten female lifespan and reduce egg output.
Females have responded anatomically. Cimex lectularius females carry a structure called the spermalege, a groove and pad of tissue on one side of the abdomen that receives the puncture, apparently limiting damage and infection. Males aim for it.
Bed bugs feed for several minutes on a sleeping host and then withdraw to a harbourage, which is why bites are found but the insect usually is not. They can survive months between meals in cool conditions. Despite the bites, bed bugs are not established as transmitters of human pathogens, and the main harm recorded is irritation, sleep loss and cost.