Talk it out with him, hopefully the finding out wasn't something like he went to pay for the date with his credit card and it was declined, but I'd want to know how this debt happened, is it currently growing or shrinking, what are his current attitudes toward debt etc.
Ultimately if he's got a solid budget and plan to get the credit cards paid off, not too big of a deal, may turn any wedding planning into a super budget wedding but I could live with that (I think). But if he sees no problem with having tens of thousands of dollars in debt and doesn't believe in budgeting, living within his means, or trying to get out of debt, then yeah that's probably going to be a dealbreaker. I'm not going to be jointly financially responsible for anything with that guy.