There are so many "cooties" we are at risk of catching when we have multiple sexual partners. There are no features or properties we can observe in our potential playmates that can assure us the other person is STD free. The brightness of their smile; their apparent disposable income; their popularity at the party; and everything else are totally meaningless as an indication of what we might catch (or even be exposed to and happen to not become infected). We can't even be completely certain of our own sterility. What critters are viable on the sheets of the recently used bed at the house party?
I think it all depends on Lady Luck because safe sex is never really safe. We would be unlikely to willingly rely on the effectiveness of condoms to protect us from the known STD carrier. Condom use, is just one of the rituals we may practice to influence our risks (and our peace of mind) and I believe it may be less effective than a warm soapy shower soon after an encounter. We believe we influence our risk by selecting playmates from the routine participants at the recurring house parties; we imagine they would not knowingly propagate an STD because it would damage their reputation.
I'm having difficulty assigning my personal value judgment on your data. On the one hand, the studies need to be done and I'm glad you're actively seeking answers. The good news is that your data shows swinging as a reasonably low risk activity. The unanswered question is how can we individually use this data to support best practices in our swinging choices to manage our risks. Can we demonstrate the merit of any particular ritual?