I'm not terribly sure I understand your issue, so I shall phrase it as I understand it, and answer anyway incase it happens to be useful.
Problem
Given a mass on a smooth inclined plane, we would then draw three arrows: The weight of the mass, the reaction force of the plane on the mass, and the resultant force (also the direction of motion) going down the plane. The issue we have is that by Newtons Law, we are expecting the reaction to be opposite to weight, although it clearly isn't for an inclined plane. Why is this?
Answer
You are not considering the correct pairs of forces! When we look at weight, this is the force due to gravity the particle experiences. The reaction force we know to exist by Newtons third law, is the weight of the Earth as according to the mass! (which is equal in magnitude, but opposite in direction). The way to think of this lies in the symmetry of the situation. The earth is attracting the mass, so the mass has weight according to the earth. Conversely, the mass is attracting the earth, so the earth has a weight according to the mass! In our situation above, we only talk of weight from the earths perspective, and so we are ignoring the (newtons third law) reaction force.
The reaction force on the plane is a result of the weight, but is not the reaction force to the weight of the mass. The mass is unable to move through the plane (obviously.. it is in the way!). It can only do this, though, because the plane is providing a force to stop the mass moving through the plane. In some situations the plane cannot provide enough force to stop the mass going through the plane.. in this case, the plane breaks! The reaction force is just the force the plane has on the mass to stop it penetrating the plane; and this is equal and opposite to the force the mass is trying to penetrate the plane with. Which will be the component of its weight normal to the plane. The weight is not the force the plane opposes, it opposes the force on the plane the mass can provide because it has weight. It's not as direct as simply "the weight".
I hope this helps; but please don't hesitate if you'd like to ask more questions!