Maybe he's one of those "mentalists" (people who do mind-based tricks and illusions on people like "cold-reading" to the point that they seem like psychics, albeit their methods are based on scientifically-proven psychological and sociological methodology instead of the inherently supernatural - think of those high-res Las-Vegas magicians) and he was just able to pick up the fact that you were bisexual and polyamorous simply by "cold-reading" you and the like - who knows? I know how to do some basic "mentalist" stage-show magic myself, too, as well - I know how it works!
The apparently popular-in-this-comments-section "gay-dar" theory may as well be equally plausible, too - your cousin is truly of the peculiar type, that's for sure as far as we know!