I've always thought that the members of the Gryffindor Quidditch team would date each other, just because spending so much time together would create a special sort of bond that cannot be forged through other sorts of activities. Fred would be with Angelina, George with Alicia, and Katie with Oliver. (Harry being the exception; but then again, Ginny was later on the team as well.) Everyone just seems to fall into place.
Katie loves Quidditch and Oliver loves Quidditch, so again, why not?