When Father sacrifices his five subjects—the five who have all seen the portal—why is it that they are all fine right after? I would have expected them to be gone, or dead, or something.
Short answer: There is no canonical reason for this.
Long answer: In English, a sacrifice is defined as:
an animal, person, or object offered in an act of slaughtering to God or to a divine or supernatural figure
However, a sacrifice in the Fullmetal Alchemist Brotherhood (and manga) canon is simply a being which has attempted Human Transmutation, and thus passed through the Gate of Truth. This sort of "activates" their Gate, so that someone with the right knowledge and setup (read: Father) can access it.
I think the Fullmetal Alchemist Wiki says it best:
Contrary to what the name suggests, the Human Sacrifices don't actually die and survive with no ill effects. In fact, the Sacrifices were some of the few humans to not have their souls dragged from their bodies.
So, why are they called sacrifices? It's really anybody's guess. In a way, they line up with another definition of the term:
an act of giving up something valued for the sake of something else regarded as more important or worthy
In that sense, they are sacrifices; just not in the way we would typically think of a ritualistic sacrifice taking place.
So, no, no particular reason why they don't die, except... well, the author needed them alive.
For the same reason none of the people in Amestris die: Van Hohenheim's plot to reverse the circle. While this isn't specifically mentioned in canon, it does logically follow.