I would firstly say -
My wedding is not a random party, it has cost me time and effort and money and therefore we have chosen our guest list very carefully. Whilst we want our guests to have fun we also want them to understand that is our wedding not a couple of drinks and some free grub for you and random people we don't know.
Secondly, to parents, you have one wedding under your belt which was a shotgun wedding arranged in three weeks after your father found out you were pregnant, that was held over 50 years ago, how does that make you an expert on my wedding. I arrange conferences and events on a daily basis yet every last decision of mine is scoffed at.
And finally
Our wedding is a personal thing to reflect us, not you random guest, please see the first point, this is about me and the groom not about how good a day's free food and drink we can provide you with.