QCTM
Now that you have done it, YOU are the expert. Seriously, with 400 lbs of people on board the C14's center of gravity is pretty high, and is also shifted rearwards now that you have (had) a passenger. That means wheelies can come fairly easily. Depending on incline, weather, rpms, etc - it can surprise you how readily the front will lift.
Lots of passengers feel a bit insecure without a backrest or trunk. Add some acceleration, nose lift and a bit of surprise, and you have the recipe for a less-than-happy wife. As you know.
No real solution, but some things may help: Listen. Apologize (no excuse or explanation, just apologize). Flowers. Dinner. Backrest/trunk.
Good luck.