Husband is probably gay or bi, and is possibly cheating.?

My husband and I have been married for over seven months. Over the past few months, I have noticed changes in him. He rarely ever wants to have sex with me, he is always staying late at work (we live 45 min away from his work). Its always, im hanging out with a buddy, or helping a fellow co-worker. He has anger issues and PTSD and we have had a few blow ups, but our last one he really opened up to me and admitted he needed to change. We have been doing amazing since I thought. Well he left an email on the computer after he went to work and I read it. It was a response to a craigslist casual encounters ad, and was a man. They shared face and genital pictures and talked about what they wanted to do to each other. We are going out of town this weekend, and he even emailed this guy telling him when we would be home and wanted to meet up then. I looked at previous history online, he frequents craigslist personals, alot, responding to many male and female ads. I am very hurt. He tells me how much he loves me and talks about our future children and our life together. We are even going to meet his family in another state. I dont know what to do. What would you do? Would this be grounds for divorce? Im so hurt and confused.