One of the keys to a lasting relationship is to learn to fight fairly. Everyone is going to have times in their marriages when a disagreement comes up. You both need to learn how to handle those times constructively - instead destructively. Your boyfriend walking out in a rage and heading off to blow off steam isn't all that bad in itself - assuming he wasn't out having sex with some woman. Sometimes you have to get out and cool down before you can have a constructive conversation. The issue here really isn't that he went out and partied without you. The issue is marriage. If this is the one and only time such an event has happened, it sounds like you have a good man. Nobody's perfect, and we all have to blow off steam once in a while. Sometimes we do that in ways that are pretty stupid. Learn from it, and move on.
Think of his position too. It appears he's been wanting to marry you for quite some time, and YOU are the one who's been putting him off - for three years! What kind of a signal do you think you've been sending him? In this day and age, a man who really desires marriage is almost a rare animal. This man sounds worthy of doing what you can to repair the relationship. I don't mean to say you should be a doormat either. Just try to put yourself in his shoes for a while and try to see things from his side. Pre-marital counseling would be a really good idea too. :-)
I am assuming that you two have not talked marriage since the time he asked you to marry him and you declined. I you know you are ment to be with him then why would this not have been talked out since then. My suggestion would be for you two to sit down and talk and set a plan in motion. If you want this to work then you need to discuss and not fight over what you both want. The keep thing to remember when having this talk that you never start a conversation with the words like you did this and you did that. By doing that, you are putting the blame on one person and you will both end up losers. I would suggest you approach it by saying something like, what can I do to make this situation better. You will get much better feed back and then no one has to be a loser. Because you may when the fight but in doing so you are hurting the person you love.
Well I can see how upsetting that would be, him going out and being so sneaky. But do you trust him? Do you feel in your heart that he would never do anything wrong behind your back? It seems to me, (As an outside perspective) that he is being quite immature about the whole thing. Why has he changed his mind about getting married? I just think that either he has some stuff under his belt that you are not aware of, or he his just being lazy and wanting to do his whole thing. Sit down with him, talk with him, and tell him your not playing anymore games. I mean, you have children on top of everything. he affects them just as much as you. Stand up for yourself lady!!! Good luck, and keep me updated please. Have a nice day.