I think it's bang out of order. People should pay for their own weddings (or the parents of the bride or groom if that is their wish). You shouldn't be asked to contribute, unless, as cupcakes said, they were poor and you were rolling in it. The answer is no. I wouldn't even give a reason. If they are so unreasonable as to ask in the first place then I think any reason you did give would be thought to be inadequate, anyway. Stand firm and good luck!
P.S. I once helped someone out with wedding costs. I wasn't rolling in it by any means but wanted to make a gesture. The item I helped to buy was later sold and I wasn't offered any share of the proceeds, then or at any point later. I put it down to experience but was only moderately generous at that same person's second wedding. And now I think of it, no, I didn't get a thank you the second time, though they definitely banked the cheque. If they marry again, I doubt I'll go to it or send a card - sod 'em!