When invited to an event, should you ask who else will be there? Please read description.

I was invited to a baseball game by a friend. I never thought to ask who else would be going. Yesterday came around and my boyfriend asked who else I was going with. I asked her if anyone else besides her and her boyfriend would be going. She mentioned 2 other guys. One of them has been a friend for years and the other one I met yesterday. I told my boyfriend and he then texted me later that we were done. He said I was untruthful.

I avoid going out because of his jealousy but I just don't understand how I was untruthful. I understand why he's upset. I'm just confused and I don't know how to fix this.

For anyone who's wondering, we have been together almost 5 years.