Welcome to the boards.
Poly rests on clear communication and explicit agreement. Having talked about something before is not that same as having an explicit agreement that something is or isn't acceptable. Unless you said it was OK for him to have other partners in those earlier conversations, it wasn't OK for him to strike up a relationship with somebody else. It's the whole "full knowledge and consent" part of polyamory.
Do you think he avoided speaking to you about it just so that he could present it as fait accompli to get you to go along with it? Is he always likely to do things without thinking them through? Does he really think simply having talked about something as a possibility means its OK?