I think it was rude to just walk away without an explanation. It's usual and customary to disclose information when getting to know someone, and it's not always going to be something the other person can accept. That's why it's best done sooner rather than later. I don't know how long you've known him or in what context he had 4 subs, but monogamy, like a lot of other things, shouldn't be assumed. It should be discussed.