I pay for first class and try to select my seat in order to have easy access to the restroom. I do get annoyed when passengers from Y keep using the first class bathrooms, esp when there is no reason to, other than for convenience because the aisle is not blocked. I understand there are circumstances where passengers need immediate access to the restroom and they should be provided the courtesy of using the nearest restroom, however, on some flights it seems like there is a never ending stream of passengers using the F restroom. It has been a long time since I have seen any FA enforcing the "rule" that passengers should remain in their cabin. I have also witnessed passengers from Y who after leaving the restroom help themselves to the snack basket that is left available for F passengers. It seems that the FA just do not care anymore. On a recent flight I really needed to use the restroom but before I could get up from my seat, someone from Y would beat me to it. I finally got up from my seat and waited near the galley and mentioned to the FA that F passengers should have priority over others to use the restroom and their response was there is nothing they can do. Doesn't seem to be a problem on foreign carriers.