In the history of disease, ranking high among the list of most deadly are the rogues gallery of sexually transmitted infections--syphilis, genital herpes, chlamydia, gonorrhea, HIV and especially AIDS. Combined, these diseases kill off millions of lives annually.
The curiosity about STIs is that the focus on them is exclusively human. The bonobo, a cousin of the chimpanzee, relieves tension through intercourse, yet there is no evidence of venereal ills. Lions, of course, form prides, a harem consisting of one male lion and several females. One male mammal, as far as I know, does not mate with the same female in more than one season.
Promiscuity and infidelity are rampant in the mammal class, yet humans are the only ones who suffer poorly from those acts. Before I allow evolution in an alternate Earth to tweak the human genetic structure before Out of Africa, I must ask--where do STIs come from, and why do they make sexuality a high price?