So I analyzed my messenger bag today. I wear it in front of my body on one hip, so the strap is hanging almost vertical and does not bisect the boobs. The BAD BAG I had given to me was a weird sporty asymmetrical design (could only be worn on the right) with a wide flat strap, the bag part was actually a backpack IIRC. Come to think of it you mostly see that style on men, I guess we know the reason!
I actually prefer wearing the messenger in front (vs on the side of my hip) because I'm more aware/in control of it when I ride the super-packed subway. On my side it gets bumped a lot by people trying to get by me (and frankly I don't need any added hip width!)
I am totally familiar with hitting myself in the head with bags... mostly when bending down to deal with small kids' needs but yeah, low grocery shelves are also an issue.
Suz, you are too funny

Haha, I thought you guys might be amused to know that I hit myself in the head with a purse again today. This time I had a big, heavy leather purse on my shoulder and I bent over to put on my boots, and CONK. Ow.

Diana, please no concussions before your wedding!