The short answer? Yes. It makes you look discourteous, as essentially all functions in the adult eventsphere allow you to bring a date. Yes, weddings aren’t exactly cheap in today’s world, but it’s rude to set a guest’s terms in such a way if you want the guest to come to your affair. It’s natural to assume the guest will bring someone, and if they don’t, that’s just fine. What’s important is giving your guests the choice.
If you aren’t sure whether some of the people on your guest list will be bringing guests, call, text, or email them. You may also make it clear on the invitation that if the person is bringing a guest to let you know asap. Should you find your wedding guest list is on the longer side, look for ways to trim the Big Day fat, such as hiring a friend to act as the DJ or offering a buffet.
The bottom line? If you care enough about the person to invite him or her to your wedding, you should care enough to allow for a date. Do what you need to do to make it work and remind yourself why you want these people at your wedding. This is one of the biggest days of your life, after all!