I probably would have gotten huffy too if I was in his position... I'm sure he's aware that most poly people aren't interested in cheaters. Why even bother messaging him if you weren't interested in him? To me, that just seems like going out of your way to judge someone when it really isn't your place. He wasn't asking for opinions or propositioning you personally in any manner. It would be the same as someone admitting in their profile (or alluding to the fact) that he/she is a kleptomaniac and has a tendency to steal things and someone sending him/her a message just to tell that person that stealing is wrong and that they have no business being like that.
Maybe he and his wife have a DADT policy and that's why it needs to be discreet. Maybe he just likes recreating the "sneaking around" adrenaline rush even though his wife is okay with it (I've done that). Maybe he has a VERY strict work environment or is very well known in public and can't come out as poly. There are many reasons to be discreet. Yes, I agree, usually people who use the word are cheating, but it isn't a hard and fast rule, and I really don't think it's proof enough to send a message out of the blue.
(I apologize if he DID contact you first, in that situation, I would have replied similarly. I am also not trying to be mean, just saying that I would be rather pissed if it happened to me - someone who has never cheated but has asked for "discreet encounters"-, so I understand his huffiness)