How do I convince my girl to start riding?

People have, to a large extent, the ability to exercise freewill. If you're girlfirend wanted to ride a bike I'm sure she would be capable of communicating this fact to you, or whoever else she may feel comfortable discussing it with..

People also sometimes enjoy discovering new things and can be resistant to them at first. 4 years ago, I didn't cycle at all. Now I own 2 bikes, ride 100s miles a month and enjoy it very much. It took someone else to get me into cycling and I'm very grateful.