Wedding invitations are expensive. Planning a ceremony is stressful, not to mention just plain overrated.
Why not get married quickly? You want to be together, don't you? Whether your union would be disapproved of, you have to get married before one of you leaves for a long time, there's another marriage, possibly involving one of you, you have to circumvent, you're drunk and it seems like a good idea, or you're just low-profile folks, you may find a
Then, you two will be married, and no one will know. Mystery keeps a relationship fresh, you see (that, or it keeps it very difficult, but that's negativity talking!).