Search Results: forgive my boyfriend for cheating

It was my cousin that cheated on her then boyfriend (now husband) way back in 2003. She was 19 at the time and he was 20. He caught her cheating with his friend in his house but then forgave. They worked it for years, he promised he wasn't going to bring back her cheating anymore and got married in 2009.

Their current situation:
They got two kids; a 9 year-old son and 6 year-old daughter.

According to what she's told me, for the past 3-4 months, he's been creating random arguments, has brought back her past cheating from 2003, has insulted her in front of the kids (calls her the C and W words), goes out clubbing and returns home at 3am drunk, has his cell set on private and even said that if he cheated, it would be even and that she would have nothing to complain about. Is he somewhat in the right in his logic?

My boyfriend and I have been together for +4 years. In the beginning (first 1.5 years) he cheated on me multiple times. I forgave it all except for one time where he had a whole relationship with another. I still couldn't leave him alone (because of love) and decided to check his Skype (I knew his password). I know I shouldn't have gone on his Skype but I just had to break my heart to be able to leave him.

After months we started talking again. He didn't speak to the girl anymore. From there on we are dating again.

Now, about a week ago he started talking to her again. He offered me to read the messages between them but I declined. I was terrified that I was going to get upset and cry in his face. When he was sleeping I broke into his phone. Obviously I should've taken the chance he gave me but my feelings took over (again).

What I read was honestly heartbreaking. He told her all kinds of sexual things like:
"I wanna come give it to you.", "I would be able to have fun with your small boobs.", "You're allowed to use your mouth sometimes too.", "Come sit on me.", "I'm your BFF who wants to fuck you sometime."
I left the bed and cried until the sun came up.

When I told him, he just told me it was a joke and this is what they've always been like. I was hurt so I didn't accept his kisses/hugs. In the evening, I was alone, still crying and upset from what I've read. He was in the other room talking to his friends which included the girl. I just can't understand how he thought that was a good idea? I told him this too and he said it was a coincidence. He also told me he did it because I pushed him away.

I tried to forgive him and yesterday I was doing my absolute best to make it a good night. I kissed/hugged him, played PS4 with him, etcetc. But I still had this feeling, so I did it again. I checked his phone and he was complaining about me to the girl.

Am I just toxic? Is this normal? He acts like I'm upset over nothing, is he right? Please help.