There's a difference between poly and open. Which one you are determines how over the line you were. If you're just open, then you're guilty of not letting him know first. If you're poly and you've determine that any sex had will be within an established relationship then you'd be wrong for 1) not telling him you were interested in this guy and 2) sleeping with this guy without officially being in a relationship.
Either way setting people up to fail like your ex did (you can sleep and have relationships with other people but if you do I'll dump you) is wrong. Dead ass wrong.
If I read what you wrote correctly he gave you the option of being open but didn't actually expect you to take it. Lame.