I understand and agree with the logic that the higher the number of anyone's sexual encounters, the more frequent the exposure to risk of STIs. This can be further exacerbated by exposure in areas of increased risk, i.e. sex with IV drug users, for example.
However, this could be applied to any risky behaviour:
The more often one drives, the more likely one might be in a car accident.
The more often one eats fast food, the more likely one might develop diabetes or heart disease.
The more often one takes public transport, the more likely one might catch a cold.
I think what I am trying to say is that people try to assess levels of risk every single day, and everyone's assessment of or tolerance for risk is different and can be influenced by numerous factors. STIs are an unfortunate example that often come with shame, stigma and judgement attached, so people react more emotionally because it feels deeply personal.
For comparison, I was once mugged and punched in the face while walking home at night. My boss said "Well what did you expect, walking alone at night?" and everyone roundly agreed that he was an arsehole.
When I confided in a friend that I had been diagnosed with herpes, she said "Well what did you expect, sleeping around?" yet somehow in this context, her response was acceptable (not to me, we are no longer friends).