In this scene, we find out that Oberon and Titania, King and Queen of the fairies, have been romantically involved with Theseus, Duke of Athens, and his fiancee, Hippolyta.
We are told that Oberon was having an affair with Hippolyta. Titania scolds him for it in this scene. When she scolds him, Theseus responds in kind. He tells Titania that she should not be talking about his affair because she was having her own affair with Theseus.
So it seems that the fairies have managed somehow to end up having love affairs with the human beings.