I don't know how you lnow - the only time I saw my destiny appear before me, and it happened in a nanosecond, I just knew - it was so intense I almost collapsed. It didn't become physical for a long time, though the sexual attraction was implicit from the beginning. He knew too, just as immediately. And it led after a long time to total disaster.
The relationship that grew slowly and included physical and emotional and intellectual elements is the one that lasted. Don't know what that proves, except that deep in my mind i have a tiny niggle, less from the movie than the story, where AP says every time they met there was " the brilliant charge of their infrequent couplings", that the physical thing was a strong part of it, and that's what proximity, familiarity and routine often erodes. I wonder if, whether they'd been together permanently, the physical thing would have burnt out and if so, what they would have ben left with.
Put this down to me being old, wqrinkled, cynical and needing a chocolate fix.