I have to play the devil's advocate a bit here...affairs are not things that ever end well. I'm no fan of monogamy, but there's a reason cheating on one's partner is highly discouraged. In such an emotionally charged situation, this may in some ways be the best solution; as nice as a calm conversation would have been, it's probably not likely.

I think you're very proper in feeling hurt, upset and angry; you have cause to feel them all. But temper that with the knowledge that it shouldn't be entirely unexpected, particularly with a couple warning signs you've mentioned.