Relationships are about a lot of things. Taking care of you and the kids is just one aspect of it. It seem to me like for you, that was enough, as long as he held his financial and emotional relationship to the kids, you were happy, or at least content with not getting the attention and affection that are usually associate with the role of girlfriend.
People make different situation work for them. It's very hard not to get judgmental over your boyfriend and you. Honestly, I think he's a mess and you have a very low self asteam to live in an impossible emotional state in which your guy can have sex and is at risk of leaving everytime he meets an attractive woman.
Marriage will not prevent this from happening again, I think that now, he's hurt from the other girl cheating and want the comfort and stability of marriage, but after his ego recover and he meets another girl, he will have sex and maybe leave again.
What would help?! I'm not sure, probably you working on yourself and getting more confident through life, understanding that you deserve to be loved and to have the stability of a fully committed partner.