Yes, talk to the office manager.

Take your Policies and Procedures manual in, with the sections that say you aren't harming the dress code highlighted. Then point out the women in the office who wear low cut tops and stretch pants and say something about a discrimination and a lawyer. Yeah, and you can mention the health benefits of riding a bicycle, how you are valiantly sacraficing your share of gas so that they can drive to work. Then say something about a raise.

Seriously though, a pair of jogging shorts might solve the problem.

-Rob.