When two animals are in love mode they walk towards each other until a baby is spawned. If only one animal is in love mode, nothing happens.
However horses try to breed eventhough the "partner" horse wasn't given a golden apple/carrot yet. They will still walk towards that horse and pursue it.
Steps to reproduce:
1. Get two tamed horses.
2. Give only one of them a golden apple/carrot.
Observe how the horse you put into love mode now tries to breed with the other horse which is not yet in love mode. Try the same with two pigs, nothing of that kind happens.
Confirmed.