Since you asked what I would do if I were in your situation: yes, I would leave. You have spent enough of your life with someone who does not make you happy. I think it is very telling that you have been together for that long and have never gotten married. Unfortunately, as honey pointed out in the last group therapy about marriage this means that the laws regarding alimony, etc...are very different then they would be if you had been married this whole time.

Think about it this way, life is short and you could die tomorrow. You deserve happiness and the feeling that your partner is 100% committed to you, (although I have a much more liberal and different perspective than you do about the porn thing). Personally, I would never stay with someone that has cheated on me or that continually lies to me. I think at this point you should contact a lawyer to determine the financial options available to you. For example, will you be able to get any alimony from him even though you were not married? I also think that you should talk to the counselor before taking any action. Normally I would suggest couples counseling but it seems like in this circumstance you have pretty much checked out of the relationship and have a lot of contempt and resentment for this man. So, if you just want out then talk to your counselor about that. I think that you will be able to trust a man again. This time, you will be able to trust a man that is deserving of your trust. But in my honest opinion you might want to lighten up about the whole porn thing. Every single guy I have ever gotten to know has watched porn. Including my dad, my guy friends and every single guy that I have dated. Guys like their porn. Not saying it is right or wrong or that I personally love porn or anything, but it is what it is. Guys are much more visually stimulated than women are. I really think that you will be ok on your own. How are your financial prospects? Start saving any money that you can right now in your own personal account that he does not know about. Make sure that you set up the monthly updates so that they are accessible online only. At this point, you have to protect your financial future. Lean on friends and family to get you through this difficult time. You will be ok. Good luck.

Seeing a counselor is a good idea. I don't know about how badly you feel with everything. If I could give any advice it would be this: Make a decision in a few weeks. Don't drag it on for another two years or three or four undecided in an empty shell of a life. Also get as much information as you can from friends and a good lawyer if things go in that direction. Being alone is hard, but aren't you sort of alone already with that cheating husband? What kind of security is that? Good luck to you.