As a speculative fiction romance writer and reader, I've seen many a book where the heroine's intrinsic scent is what clues the hero that She is his mate. That's kind of romantic, no? But it's not something common to our olfactory abilities. A shame really, if you think about it, because it could aid us in selecting a more physically appealing mate. I'm not talking about his or her looks. Physical appearance changes due to time or accident. What I'm talking about is something I've seen in the movie, Species, and which has been determined to exist in mice. If you recall, in the movie, the alien hybrid who was desperate to mate and procreate had a willing partner but something in his scent warned her he had a physical disease (diabetes, IIRC) that would not make him a prime candidate for fathering her offspring.
Well, recent studies have shown mice to have this very same ability. Apparently a receptive female mouse was put with several willing male mice who were inbred. The female wasn't receptive to their advances and the scientists think it was due to that inbred factor. You can read more here.
Maybe not as romantic as knowing a potential mate is yours but I can see a wealth of story ideas in something like this anyway. What about you?