Really? Shorts are masculine are they? Funny. I thought trousers and shorts were unisex.
Yes, trousers and shorts are generally considered perfectly acceptable for both sexes. All well and good and as it should be.
I don't think anyone's disputing the fact that skirts are not currently widely accepted as unisex in this part of the world, are they? They're clearly perfectly practical but the social pressure of the skirts/females/lesser stereotype means men tend not to want to wear them.
Which means skirts are still being associated with females and not with males. Which is literally gendering them, right? Attach a load of damaging and oppressive stereotypes and boom: you have the seeds of patriarchy.
And males still get unconsciously associated with trousers simply because they're so rarely seen sporting a one-tube, which is, again, literally gendering trousers. It would probably be more accurate to say men as a class are still trying to gender trousers by refusing to be associated with skirts.
Definition of gendering I'm using is here is basically "setting up this unisex option as for girls and that one as for boys." Which definition is everyone else using, out of interest?