I have no idea why the idea of writing a post on port-o-potties popped into my head but I think I was talking to someone about them recently.
If your wedding will be held outdoors - at a park or campground - or at a private home, it is important to make sure that there are adequate restroom facilities available for your guests. A park may have 1 or 2 public bathrooms and a private home may not be able to handle over a 100 users in a six hour period.
There are many companies that you can call to rent portable bathrooms and you are no longer limited to the all plastic, no flush variety. Portable restrooms have gone posh and companies are offering loos (plural of loo?) with vanities and a place to hang a lady's purse.
You can search online for companies you can rent portable restrooms from or talk to your wedding planner or florist. If you're getting married outdoors, assuming you have a tent rental company on hold in case of rain, they may be able to give you some recommendations too.
Here's a short list of some companies I found with a quick Google search:
Mr. John in New Jersey
Elite Portable Restrooms in Louisiana
Potty Queen in Pennsylvania
Potty Finder has a search by state page.