The rule is that Superman gets with Wonder Woman by process of elimination. It is that Superman's other possible love interests have to die before Superman gets with Wonder Woman. This was broken in JLA Act of God when Kal gets divorced, or The Dark Knight Strikes Again where they have been having an affair since 18 years before, and the other love interests are not dead until the end of The Dark Knight Strikes Again when they get crushed and burned to death when the Entire Daily Planet building collapses with them inside.
Then, they had a relationship in the N52 with other possible/previous continuity interests still alive while it was going on.