The burden is only put on you if you want it to be. If you end up paying for it and don't like it, you have only yourself to blame.
As others have said, my wife and I paid for the majority of our own wedding. Our parents paid for some, but we didn't ask or expect them to. We will do what we can for our kids, but won't be taking out loans for their weddings or honeymoon, I can assure you of that.
Your son and future daughter-in-law should be the ones to figure this out and plan a wedding with the means available to them. Nice weddings don't have to be expensive.
As far as your son owing you 15K, thats a separate issue. I've always gone on the assumption that any money given to family members is a gift. If they pay it back, great, but I don't loan family members money with the expectations of ever seeing it again.
