Having read your thread I'm really sorry to hear that he is still doing this.
You have obviously invested a lot of time in your relationship but as others have said this is not a reason to keep it going. A friend of mine ended a 8 year relationship after he started straying. He realised that this was a sign that it was over and that he didn't really care now. He ended it and now is happily married and has a lovely wife and daughter. They are so well suited to each other and it was clear that he wasn't with his ex but the vast amount of time they spent together was making it impossible. How ever some alcohol and pretty women later and he'd started the process that ended it all. Not thats the healthiest way do work things out though!
What he's doing is cheating on you no matter how you look at it. I wonder if like my friend he is perhaps unconciously constructing a situation which gets him out of having to ends things.
I personally don't have a romantic view of the world. There is no such thing as your perfect partner. Given the time and resources I bet we all could find 100's of people around the world who we could find sexually attractive and fall in love with. It is only society that has imposed monogamy on the world and I think biologically men have an unconcious drive to procreate ( ie have sex ) as often and with as many people as possible. It is only our higher brains and our love and sense of respect for our partners and guilt that normally prevents us from straying. I think this is the reason 1 in 3 marriages fail in the UK. That and the fact that people think they can change someone. People only ever change when they want to.
Consider this fact when your thinking about your future. Pushing on and hoping it'll get better or worse yet getting married anytime in the next few years would be a mistake in my opinion. But i don't really know you or him and what you have together as no one else can now that but you.
Ultimately have respect for yourself. There are a few sentences in your posts where it comes across as if you're unsure of your self worth. This is important to sort out for yourself as valuing yourself is a good starter for a healthy relationship. No I haven't got this from self-books but from talking to people who have long stable marriages or relationships. These couples are great people with interesting lives apart from each other but then have space and time where they have a good life together. Like the 3 points on a triangle someone once said to me ( I had to think about that for a while! )
There is a very interesting point in a book called 'Blink' that researchers were able with the help of very few bits of data and 15minutes of video of a couple arguing, they were able to say with over 90% accuracy if that couple would still be together in 15 years time ( by checking again with them after that time ). The theory was that small problems in communication very slowly eat away and build and build and over time until one day a couple find they have no love or respect for the other person anymore.
Sorry for the last bit but that information blew me away that it wasn't actually hard to predict a couples future in 15 years, something I would have though to be almost impossible.
I wonder how much respect he would really have for you if you go back? Why I say this is that you have threatened him once already with an ultimatum and if you don't follow through he may take that as a green card that he'll probably get away with it again. May not be now, he might stop but what if an opportunity with another woman presents in 2 years, 5 years, 20 years? Will some bit of his brain be telling him he'll ultimately get away with it again?
I suspect amongst all the noise and the arguements going through your head there is a quiet little voice telling you what you need to do. Do not doubt this voice. Listen to it and follow it.
All the best no matter what you do.