[bitshiftfaced]

Each person carries some chance of carrying an STI. Some people aren't aware of their status or may deliberately hide it. For those two probabilistic variables alone, while the absolute risk may be small, it's still at least double that of 2-people relationships. If someone is more likely to enter a 3-person relationship after leaving another 3-person relationship, you have added network effects increasing the risk. How could it be true that the risk isn't different? What have I missed?

[some_furry]

You're assuming ceteris paribus, but that isn't a given.

People in poly relationships tend to know their status better than monogamous cohorts. Thus, even if one partner has an incurable STI, they're usually aware that they have it, and keep their viral load undetectable (which prevents transmission), so the factors round the risk down to the same very low range, despite more people involved.