I had a similar deal going with an old school friend when I was in my twenties. If neither of us were married by the time we were thirty, we'd marry each other. Trouble is, then I dated her when we were both 28 and it went miserably, and we've barely spoken since!

Sounds like a half way decent plan you've got there... Maybe we should start a commune and have like a mob of unruly unwashed children running about...