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.
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.