It does carry a higher risk of transmitting an STD from one person to another, yes. So I'm wrong to say that it has "nothing" to do with it. But in terms of epidemiology, the higher risk of transmission associated with a given form of sex is insignificant compared to having many partners. Spread, not risk of transmission, is by far the dominant factor here, even if they're not entirely separable. Sort of like the difference between scalability and performance.