But I'd have more of an issue with those men who have had multiple ONS, often dozens, and think this is more of a negative character trait than the person paying. And certainly, more dangerous to sexual health.
The ons's would (presumably) involve consent, unpaid consent, real consent . With women who are not likely to be coerced, addicted, from impoverished countries etc etc.
On the STDs front, some prostitutes have intercourse without condoms, lots have oral sex without condoms. Lots receive oral sex without dental dams. Some perform analingus without protection.
Some STDs are spread even with condoms.
A ONS person may have sex with multiple partners around the same time, a prostitute is almost certain to have sex with multiple partners around the same time.