Should I marry him?

Hi guys. I'm bisexual and living in London. A couple of days ago my ex boyfreind Dave showed up at my flat and asked me to take him back to which I agreed. He then asked me for his hand in marriage to which I again said yes.
We were going out for two years but in the summer I ended our relationship following his infidelity. I don't blame him for this as I was ignoring him at the time as I was preoccupied with my education. Hes bi too and he slept with a girl who is now pregnant with his child and due to give birth soon. Initially I was very exicted about all of this but my freinds have been less than enthuastic. They're telling me to see sense and leave him as this will be a huge mistake.
We've been through alot together. I'm 18 and hes 24. We meant through a prisoner pen pal scheme (he was inside for arson and drug smuggling). I fell in love with his use of literature and his love for the full stop. After he got out we started seeing each other and we were happy for quite some time. Hes had a tough life what with prison and coming from a big family living in poverty. He is one of eight kids. He has promised me he has cleaned up his act now and that he is drug free. However, he might be going back into prison for another year in July following assualt charges therefore hes wants us to get married as soon as possible. The girl whose having his kid is a bit of a deadbeat (fat, doesn't wash, dumbass, can't read etc). Shes more than happy to give Dave custody of the kid. It's going to be a girl and Daves decided to call it Rainfall. I think we could be happy together. Daves futures looking brighter depsite the assault charges. Hes got a job in London to show me hes commited to us. He'll be cleaning toilets for $5 an hour. I would love for this to work out buy my freinds say it never will. What do you guys think?