Stacking disadvantages

[MunchkinMan]

For completion's sake, this is also stated in the rules:

Originally Posted by The Rules, Asking For Help

[I]f you are facing the Drooling Slime and an Elf helps you, the monster’s combat strength is increased by 4 (unless you, too, are an Elf and the monster’s combat strength has already been increased).

What evikour said is also true. The best way to describe it is this: If a Monster has a +2 bonus against males, Wizards, Clerics, Elves, and Halflings, then that would mean that there is a +2 bonus against males, a +2 bonus against Wizards, etc., and each bonus would be applied once if at least one of those types of characters was involved in the combat.