Are we supposed to believe that Harry and Sally were once satisfied with friendship, or that they always harbored romantic feelings? Option 2 seems more likely: Their friendship was actually a courtship all along. Like Harry and Sally's friends, the viewer expects and wants the couple to get together. And even first-time viewers with the most basic understanding of plotting must realize that, narratively, romance is inevitable.
The upshot is that truly platonic cross-sex friendships appear to be easier in real life than in Hollywood movies. Link