Ok. I have a Timbuk2 bag, a messenger bag, and I recently got some criticism for carrying one.
One: It's not a purse. Two: I don't have makeup, millions of receipts, gum, wallet-size photos, trinkets, or any feminine nick-nacks in it. Three: I don't have it with me all the time. Four: Contents: Books, computer. Five: The Pony Express used them.
In closing, I will submit that only one of the two ways of wearing the bag looks feminine. If you don't wear it across your chest (See Jack above), but wear it only on one shoulder, you are more likely to be accused of carrying a purse (see below). That is all.