If you peruse the comments of the Slow Bicycle blog, you'll see that several people think that accessorizing one's bike should be suggested to all aspiring to slowness, including me. I think that fenders are a must, but everything else is up in the air. Maybe some cyclists like wearing those reflective thingies around their pant-legs to keep their cuffs from getting dirty, like garters that everyone can see. I can't say.
Maybe some cyclist prefer to carry bags instead of using a bicycle basket. I can totally understand that because I love handbags, but I think that as a cyclist adds variety to his/her routes and destinations, a receptacle mounted on one's bike is going to be needed. A bag-loving cyclist could always opt for panniers-- they're like bags for your bike. Maybe you could buy more than one set and coordinated the bag your carrying with your bike's panniers. Or not. I figure that most messenger-bag-sans-bike-basket cyclists would be guys. That said, here are some mostly unisex bags.
This is one of the least expensive bags--you can get 20% off the price using code SRTN.
This TRKFLD bag is expensive, but they have unique patterns and they're long-lasting.
This is an economical choice (especially when it goes on sale) from Urban Outfitters. I love plaid; it's classic.
I didn't even know LeSportsac made messenger bags. They have a line for men too, but it's pretty bland.