Unless your pants are made of steel wool or are studded in metal I don't see how your MacBook could be scratched up. Cotton Jeans or slacks shouldn't do it.
Of course, if you have unusual body peircings in your thighs you may want to consider that as a possible cause for scratches.
Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 5_0_1 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A405 Safari/7534.48.3)
I think it can if you are wearing Jeans or something scratchy. How do you post a question??