If they were an "old" flame...then they have that title for a reason. My mom always told me, "when someone shows you who they truly are, believe them."
By this I mean if someone shows that they will cheat, or that they aren't interested in having children, or that they get drunk and make a fool of themselves & embarrass themselves & you....BELIEVE THEM!!! take that moment that you question your relationship with them as a gift from God & part your separate ways. Don't marry them hoping they change or have a baby with them hoping to fix your relationship. People are on their best behavior when you are dating & when you get married...the oil & water will separate and you will see that those moments when you questioned their character were actually more relevant.
This being said...I would react by reminding myself & my old flame that things were not healthy between us, and just because its "taboo" or exciting because it seems new and forbidden, doesn't mean that we won't realize how crappy things will get at the same point in the new rekindled relationship.