The question is flawed. If humans died after having sex, then there would be no humans...because the first generation of humans would have died out, both male and female. If men died after having sex for the first time, and women lived long enough to give birth and nurse, then we would never have evolved the capabilities to understand that reproduction wasn't our sole purpose in life. There would be no culture or society. There would be no religion or science. There would only be a bunch of puberty-aged bodies lying all over the place...and toddlers fighting each other for mates.