Family can act weird about the oddest things. I think the property issue is just one way they can assert some control over the situation and register their displeasure. It is not rational, just a reaction. I am not sure what the best response to this is. Maybe play it off as no big deal?
It is good that this does not affect your son. Maybe once they see that they are over reacting, they will start toning down some of their actions.
That is interesting about them accepting it more as a "v" if it were a woman instead of a man you were datng. Maybe if it were a woman, you would be exploring your bisexual side. With a man... well, now you are just being redundant...?
Hang in there. I bet things will settle down over time and get better.