A most interesting topic, and one worthy of continuing. The conversation spurred me to spend time reading though some NIH and CDC publications, and then some research on Google Scholar. Mind you I am not a health expert, but the data is out there in the public domain for all to see.
I feel the need to be frank and direct on the topic at the risk that some will be offended. So I am sorry to those whom I may offend if it means that what I share helps someone to NOT contract an STD.
Regarding studies and condom usage, I agree that condom usage is not likely to be 100% effective. The studies are imperfect and are based on probable statistical models that are as accurate or inaccurate as the mathematics used and how the data is interpreted/presented/accepted by the scientific community. After all, how can one predict human behavior in all cases in these studies? Regardless, I also agree with others here that using condoms makes sense with new encounters. To use an analogy, why walk the tight rope without a net?
I would like to add that there are measures one should consider beyond condom use for HIV prevention: pre-exposure prophylaxis (PrEP), which has been shown to be 99% effective in preventing HIV transmission and 100% effective (with a 95% confidence level in the data) if the partner is HIV+ and is viral suppressed. Don't take my word for it. Look for yourselves at https://pubmed.ncbi.nlm.nih.gov/31184746/ and https://www.cdc.gov/hiv/pdf/risk/art/cdc-hiv-art-viral-suppression.pdf. I will caution that some research suggests shots (typically given quarterly) for PrEP have a lower efficacy versus a daily pill when adherence is 100%. I also caution PrEP may not offer protection against an HIV person who is not compliant with treatment and has developed HIV drug resistance.
I personally consider HEP-C a bigger concern. There is no vaccine (although according to the Mayo Clinic the risk of HEP-C infection is reduced if one gets the HEP-A & HEP-B vaccine) and the treatments are expensive ($95K for a 12 week regiment), with significant side effects.
The BIG take-away here is trust with those you share your bodies with. But I agree that trust + verify is important; especially if a new encounter. And as one writer stated, an STD composite test is a point-in-time when the test was administered, and is not a sure thing. For example, it takes around 2 weeks from exposure for a person to show a positive HIV test after post exposure.
As for me, the HEP-C risk is a strong deterrent. I rarely swing.