It makes sense. If it's a person you once had unprotected sex with, then it's much easier to forgo the condom. But depending on how long it's been — or maybe it need not be long at all — that person's sexual history has been revised. Plus, it's unlikely nonrelationship sex, whether it's with an ex or not, is monogamous. Nonmongomous sex is called "concurrency" in STD parlance, and concurrency accelerates the spread of STDs.
I'd like to say something smart and helpful right now, but I don't think "never go near your ex again" seems either. We might not have to teach our exes what we like in bed, but it's wise to treat these familiar partners like new ones when it comes to protection. Are you guilty of unprotected sex with an ex?