Tassy, it is hard being with someone like that. I think you could use a fresh male perspective one the whole issue and I have read your intro and other thread about your SO.
IMHO hinding porn from a SO is not healthy. I am as horny as the next guy and do enjoy porn on occassion, when I am single. But, when I'm in a good healthy relationship the last thing I want is to surf porn. I prefer then touch of a woman I am in love with and would love to never have to watch porn or masterbate again, as for me it is very unfulfilling and empty. A lot of guys feel the same way, so don't think all guys are like this.
If you really want this relationship to work I would sit him down and really try to understand his addiction to porn (if it is one). However, if he is constantly lieing to you about it and this behavior keeps repeating it will not change unless he wants it to. Also, from what you have said it sounds like you are a VERY understanding person and IMHO you are underappreciated and taken for granted.
I would not give him ultimatiums, but have a rational discussion (for the last time) and make it clear that you cannot live like this. As a woman you need to be respected, feel safe and loved. Sex maybe good, but if you are not emotionally satisfied that is NOT enough. It is very hard to move on and even scary, but this empty and betrayed feeling you have will not go away on its own.