Tradition...a couple of people have mentioned that traditionally, the bride's parents pay for the wedding. Well, in my case, cultural tradition dictates the groom's parents pay for the wedding. My FIL felt that he already pitched in once for my hubby's first marriage. Each set of parents gave us a check for $500 each. We picked up the rest of the $14,000 expense, 18 years ago. We're still married to date, and we paid in cash.
I think couples who plan to marry need to pay for their own wedding. It's only the start of things to come...sharing their expenses. Why make the wedding any different? If the parents volunteer to pick up a portion, it's entirely up to them, but should not be expected to.