In considering how fantasy versus reality operates as a theme in A Midsummer Night's Dream, one needs to look at the role of location as an important signpost in how this theme is made manifest.
The world of reality is represented by Athens, where all is law and order and everyone knows (or should know) his or her place. The opening scene in which the ruler, Theseus, must render a judgement of law is a great example of how this real world operates.
On the other hand, the woods, are a place of fantasy, of magic, and this world is open to topsy-turvy events that have an air of chaos about them. Notice the opening scene in the woods. Titania and Oberon don't speak with the same accord that Theseus and Hippolyta do. Titania doesn't treat Oberon as the ruler he is; she doesn't give in to his rightful supremacy as the male leader. This absence of law and order gives much room for the fantasy and magic that transpires in the woods.
Truly, the woods are such a magical place that Bottom and Demetrius remark on the wonder and magic of the events that have transpired there.
There is an interesting combining of these two worlds at the end of the play when the fairy King and Queen (and Puck) make an appearance at the Duke's palace after the wedding celebration. It suggests that these two worlds of fantasy and reality are maybe a little more connected that we might have expected.
Both love and dreams are the two major themes of A Midsummer Night's Dream. No less than three couples eventually marry by the end of the play; the feuding lovers of the fairy world, Titania and Oberon, also reunite. Shakespeare examines both the romantic love that dominates the younger characters as well as the marriage of convenience that unites Theseus and Hippolyta. In all cases, Shakespeare stresses the "love is blind" irrationality of the act. Dreams and fantasies are also a prevalent theme: They are the actions that spur the imagination of all humans (and fairies), a life's blood that creates new experiences with each new day.