Maybe he picked up something unusual in your response to the guys flattery? Even if you'd never cheat on your husband and weren't attracted to the guy, praise from a stranger can sometimes mean more than from somebody you've been with for 14 years. That's just human nature. You've had a decade and a half to cope with the fact that your husband thinks you are a hottie.
If there was some sort of unconscious body language response to the compliment that he picked up on, your husband may have felt that you were more flattered by some creeps interest than the last time he said something similar. Even if your husband knows that you are faithful, INTPs can see (real or not) potentially bigger patterns in seemingly small things. Since divining the emotions of others, isn't an INTP's strength he may have imagined some outlandish patterns from this tiny anomaly. I've a theory that trying to apply the scientific method to emotional issues is a factor in why we INTPs have the lowest satisfaction rating in marriage.
Also, the fact that NFs are generally WAY better at sensing hidden motives than NTs might have your husband asking himself why don't you see this guys intent when it's so obvious an INTP can see it.