Marriage matters because with two people you can share one car, including the costs. If you need a car 10% of the time you still pay for it, so you may as well use it. However if you are married share the partners car. Odds are good that even a 10% use of a car overlaps with your partners use if you don't have some other alternative, and so most buy a second car, but a bike could cover enough transport that your remaining use doesn't overlap.

I said "marriage like" relationship. Marriage as other legal and cultural connotations that may not apply, but your relationship is still such that you can share one car.