The fact is that this is our living, so we can't afford to be getting STI's. Statistically we are far less likely to be passing anything on than a one night stand or an affair where often condoms are not used. I doubt very much they would have had the same conversation if he had been out and had a one night stand. Of course they would have rowed, but she wouldn't have emphasised the need to get checked out. In fact I can not remember ever hearing anyone talking about STI tests in a soap when one party has had sex away from the relationship, it's always been about the infidelity itself.
So... sure she may get checked out, but it is peculiar to the fact he went with a sex worker that it is drawn attention to in this case and THAT pisses me off. He's had numerous affairs and one night stands before and no one ever mentioned it before.