Letterman jackets are worn, by students who earn a letter, while playing sports. A girlfriend usually wears, her boyfriends letterman jacket. It is not common to see a boy, wearing his girlfriend's jacket. There are no rules against it.
Wow man, I wld think it wldnt go over too good with your boys but maybe you just want to make a statement or your trying to score SERIOUS points with your gf but I think your opening urself up to alot if ridicule but if your up for that, go for it. But if you do, you gotta OWN it and not cave!