system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the first sexually transmitted diseases begin?
gpt
Sex is a pretty good way of transmitting diseases, since it involves lots of skin contact, exchange of bodily fluids, etc. Probably any disease which is capable of being transmitted from one person to another can be transmitted via sex. It's likely that the most common diseases to be transmitted sexually are diseases you wouldn't think of as STDs, like the common cold, or influenza.

The diseases we call STDs tend to be contagious diseases which are *hard* to transmit, so are only likely to be transmitted by sexual contact. There's nothing particularly special about them that ties them to sex.

There is also nothing particular that ties them to each other. HPV, HIV, and HSV are three different families of completely unrelated sexually-transmitted viruses (and chicken pox, in the HSV family, isn't even considered an STD). Gonorrhea, Syphilis, and Chlamydia are caused by completely unrelated bacteria, all with different origins.

SO where did they come from?  From related organisms that weren't considered STDs, either because they didn't cause human disease before they infected a humans genitals, or because they were so successful at sexual transmission they evolved away from being able to be transmitted otherwise.