I have to say don't marry him.
The way you are feeling is the way we all feel about our addicted loved ones. We all love them and we all feel like we are turning our backs on them when we stop enabling them to do drugs and have nothing change. Fortunately, I found out that I wasn't turning my back on him, I was simply allowing him to be an adult and take responsibility for his problems.
I was already married to my husband when he became addicted (to xanax) I begged, pleaded, cried, yelled, screamed, threatened, relented, snooped, etc....you name it I did it, for him to stop using. He would ALWAYS 1) cry 2) promise it would never happen again 3) just say he wanted to be normal. He faked meetings, he faked sincerity (and boy was he good at that) and he manipulated me into thinking this was the last time he'd do it. HE ALWAYS USED AGAIN.
It was never his last time until HE decided that it was his last time. He didn't quit because I wanted him to or because I loved him, he quit because he was ready to give it up. He wanted to be clean. He still is clean.
I learned detached from the craziness that seemed to envelope him and got my own sort of help. I quit enabling and quit snooping and quit basically trying to control his life and allowed him to do it for himself. I Felt soooo much better after that, even though he was still using.
I quit threatening him with leaving and simply told him one day, even though I'm not ready to leave you yet, there will come a day that I am ready and when that day arrives you'll come home to all of my things gone out of the house, our dog gone and no way to contact me personally. Fortunately, he did get clean and we are together.
Marriage can always wait. You have kids, that makes your situation even MORE volatile because him using crack endangers you and your kids saftey.
Let him figure this out. What if he isn't done using yet? He's not going to be ready until he's ready. You take care of you.
((((cherbear))))) Its freaking hard. I know.