As fultterbye says, it's a question everyone has different views on.
My personal view is that if they are in a relationship, then they come as a pair. I didn't dream of not inviting someone's other half just cos i'd not met them.
We had about 5-6 people there who we only met on the day but it was important to us that our friends had a good time and enjoyed our day, and being there with their other half was high up on the list. I'd be very upset to be invited without my OH.