Be careful Warriorqueen.
What I am saying is I would have a relationship. This would not necessarily be an exclusive relationship, ie I would not expect it to be just me and him, because I would know that this was unrealistic.
I would consider being with him only in the sense of a casual, non committed set up, no questions asked and things taken very much in the moment and at face value, while being aware of the fact he is possibly seeing another person as well (or definitely is)
It is unusual to be alright with that. I know I'd be alright with it, but I think a lot of people expect exclusivity, ie that he will choose them and no one else, and in that case you might be disappointed.
I would also say that having known him 13 years and not yet had a relationship with him, there could well be a good reason for this, and you might not be aware of it on a conscious level. Perhaps you are just feeling lonely right now?
Just to sum up, I would not have a relationship with someone who had cheated and expect them not to cheat on me. But it depends what you want from it.