Do I need to warn guests they have to pay for their drinks at my wedding?

Yes, you need to warn your guest beforehand. And warn them how much it is expected for them to pay. Some won't mind but some will. If you don't they may just walk away when told without paying. I know I would. Your guest did not order the food. Your guest did not have a contract with the company. Your guest may not even like the food. And if that is not bad enough your guest may be allergic to the food you're serving or may be on a special diet. My daughter has food allergies. After the pandemic we would go to the same sushi restaurant, expensive restaurant I might add, and they do not update their menu to say immigration seafood, immigration crab, and ect. Thankfully we asked. Because if she ordered the same thing she would have gone to the ER. The chef told us we should go to another restaurant. We did. They were shocked. Our food bill is $200 + 20% tip. Never go back to the restaurant again. And this couple weeding dinner I would have left, then whatever friendship there was would have walked off too.