Was the straight guy hitting on you in a skeevy way, or was the issue just being hit on at all? I know that some folks see hooking up at cons as a "normal" part of the experience, but the guy's got to know that not everybody does (and that not everybody would be interested in him, anyway). I guess "I'm married" isn't as much of a deterrent in a poly-friendly venue... but "I'm not interested" ought to be, whether said explicitly or not.