> There are enough non-STI ways to contract some of these diseases that I think it makes sense for even married, faithful, and generally low risk partners to get a round of testing every once in a while.
Really? Chlamydia, gonorrhea, syphilis? How can you get those without having sex?
I'm not a doctor, so take this with a grain of salt, but generally something classified as an STI has difficulty surviving outside the human body. It's not impossible, and anything that allows that pathogen to get into the bloodstream is a risk factor for the infection to spread. Things like blood transfusions, or unclean practices like needle sharing, could all contribute. Sex is by far the most common means of transmission, but it isn't the only way. It couldn't possibly be; otherwise how would the first human have become infected?
Really? Chlamydia, gonorrhea, syphilis? How can you get those without having sex?