Imagine having a Left-handed man in these lines, defence would be impossible.
http://www.youtube.com/watch?v=PnxlhajOolw&feature=player_detailpage#t=84s
In order to fill the line he'd learn to use his sword and shield like a right handed person.
I'm left handed in real life and we learn to do a few things like we are right handed because you learn from right handed people. I play pool right handed style but I grab the stick to play pool, write, eat, and do most things left handed. It doesn't make me less left handed or ambidextrous. The tasks that I learn backwards tend to be ones where both hands will be doing something important.
If I was learning to fight with a sword and shield and people would come at me with a shield in their left hand and a sword in their right I'd probably learn to hold a shield in my left hand also because that's where the pointy thing is coming from. Because I am left handed and not ambidexturous I'd probably have better control on my shield arm then normal and less control over my sword.
In short shut up. Left handed is fine and possible. If there was a grab animation and your character always used his right hand then I would say right handed is only possible but for this task both hands are used and I bet most left handers will learn it backwards.