"Sexual Intercourse with a sex worker is one of the most high risk sexual activities there are, because of the large number of partners that each sex worker has, and many clients use more than one sex worker, so it is easy to see how infection can spread and very quickly."
Theoretically correct. The more you are exposed to a risk the more chance you have of a negative outcome. However .....
Wearing condoms is surely the first line of defence, including during oral.
Absolutely correct and this negates the above mentioned increased risk factor with punting, especially as condom use is vastly more prevalent than civvy sex. As I have said many times, cover up (correctly used) for all activity and even if you expose yourself frequently to the risk, your chances of a negative outcome are very slim. Testing is of minimal importance in comparison, especially as the majority of people don't get regularly tested unless they have symptoms.
If we liken the risk to trying to crossing a very busy main road by 'dodging' the traffic. The more times you attempt it the more chance you have of getting it wrong and being hit by that speeding 'chlamydia' car.
However should you use a condom for all activity, then you are crossing that busy dual carriageway using a footbridge. Should you choose not to cover up for oral, then the risk analogy is probably closer to using a zebra crossing.
Of course nothing in life is without some risk and one day you just happen to be on the footbridge when an HIV HGV skids, loses control and hits the bridge ... but what are the chances of that?