That was the lector's most notable error this morning.
St Paul's porneuomen, in 1 Cor. 10:8, with its obvious root in porneia, is a pretty general word. Translating it into English is a little rough. The problem isn't so much that English lacks the right words as that middle school English teachers have, over the years, abused the words so badly (or discouraged their use so effectively) that their historic sense has been attenuated.
So what is it, in pronounceable English, that Paul says cost a staggering 320,000 people their lives?
"Fornication" is your best bet, provided that people understand that fornication isn't just sex, but immoral sex. Etymological purists might prefer the truly archaic "whoring" or "whoremongering," but again people need to understand that more than mere pimping and prostitution is probably intended here.
There is a legitimate argument, however, over just how much more we are talking about. Porneia is a tricky word, rendered more tricky by the gulf that separates our understandings of sexual behavior, not to mention ethics, from those of the Hellenistic world. Frankly, this gulf is so vast (and its various two shorelines themselves so jagged) that no book yet written can map it satisfactorily.
Still, "sexual immorality" is awfully vague, and -- as we we were reminded this morning in worship -- awfully easy for a well-intentioned reader to foul up. Stick with "fornication."