<human>: Can you summarize the given text in a few sentences?

So, I guess I should give some back story here. My fiance and I have been together for 4 years. Recently my fiance has made friends with the people who live near us in our apartment complex. These people are between the ages of 18-20. Not old enough to drink legally, but old enough to throw parties to 3AM and ask older people to buy them alcohol. My fiance was really into partying and being reckless when he was younger and after meeting and dating me has grown out of it.
However, this past year has been strained since it feels like he is regressing. He's been wearing his lip ring again, he's been smoking a lot of pot and been kind of an asshole lately.
I found out a couple weeks ago that he'd been texting some girl I had never met back and forth far more frequently than we text. She frequently sends him MySpace angled selfies comprised of her humongous cleavage and orc face. Of course, I confronted him about it and he claims that she's "just a friend" and that they met through work. I smell bullshit, but I let it slide because I give him the benefit of the doubt.
Yesterday, I was playing 2048 on his phone and see a text pop up from his "friend" and find out that he invited her to his work's xmas party but never bothered to tell me. This pisses me off, so I confront him about it and he claims that he was going to tell me when it got closer to when the party would be. However, he decides to be unapologetic about it and be a general douche to me for the rest of the evening claiming that I'm overly paranoid and being a bitch about the entire ordeal.
<bot>: Fiance of 4 years decides to invite some female "friend" to a party and never bothers to tell me about it. Is unapologetic when confronted.