Both navigator Presley and Ashley Williams are great men (i mean, people). Their prejudices don't make them any less worth. If they were real, I would trust them with my life.
Wrex, on the other hand? Not so much :P
And I think you all miss one thing. "Racism" towards different species in space is whole different. For example, most of the Turians think humans are lesser race. They all are racist?
Differences between all races in the universe are HUGE. And it's natural that you would protect your own kind FIRST than any other.
If you would have to choose between saving a human and a quarian , who would you save? If you would save human, does it make you racist?
(oh god, we are talking seriously about that :P)