Fat slows down the digestion (because it just sits in the stomach rather than being digested there).. hence mixing protein and fat means the protein will be digested slower.
However, post-workout is when we need fast-acting protein the most. We combine fast-acting protein (whey or egg white protein) with simple carbs to get our body back into repair mode as fast as possible.
If we add fat to the post-workout meal, the whole process is slowed down, your muscles don't get the nutrients when they need them most (the body is most receptive to protein directly after a weights workout).. therefore you're not giving your body the best chance it has of repairing itself and growing lean tissue efficiently.
Post-workout is always fast-acting protein and simple carbs, whey/egg protein with any of the following: dextrose, fruit juice, lucozade, fruit, table sugar even, hell I've seen some people eating cherry bakewells after a workout (not the best thing but the sugar is there at least!).
For me, the post-workout shake is always a treat, currently strawberry whey, 500ml cranberry+apple juice, and dextrose (plus glutamine): gorgeous.
You can add complex carbs to the post-workout meal, like some brown bread or oats but only if you're not gonna have your post-post-workout meal for a while.
And yes, you can have solid simple carbs too like white bread or white pasta after a workout, the body just wants carbs and doesn't care where you get 'em from, although the thought of solid food after squatting is enough to put me off my food (oo-er!).
But yeah.. no fat in that post-workout shake. It should also be kept to a minimum in your ppwo meal too but don't worry if you have some beef for example.
