One of the important themes found in A Midsummer Night's Dream is reality vs. illusion. Shakespeare wants to show us that there is a very thin line dividing reality from illusion and that, actually, our realities are greatly influenced by our fantasies or illusions. If Shakespeare had not included at least some aspect in the play to represent reality, he could not have shown us the connections between reality and illusion. Hence, I would actually argue that the entire play is not one entire, long dream, but rather contains both realistic aspects and dreamlike aspects.
Theseus and his court particularly represent reality. In the opening scene, when Hermia is brought before Duke Theseus by her father, we are in the presence of reality. In fact, Theseus not only represents reality, but also reason and rationality. We especially see him being reasonable and rational with respect to his treatment of Hermia. When Egeus petitions Theseus for permission to enact the "ancient privilege of Athens," which allows for a father to either kill or send a disobedient daughter to a convent, interestingly, Egeus only asks for permission to kill Hermia if she continues to refuse to marry Demetrius (I.i.42). However, Theseus, remaining just and rational, keeps the option of sending her to a convent open, as we see in his reply to her question of what could happen to her and advises her to think her decision over carefully:
Either to die the death, or to adjure
For ever the society of men.
Therefore, fair Hermia, question your desires. (I.i.67-69)
We further see Theseus acting with respect to reason when later in the play he grants Hermia permission to marry Lysander and Demetrius to marry Helena.
Hence, we see that because Theseus and his court represent reality and rationalism, the whole play of A Midsummer Night's Dream is not just one long dream, but rather a mix of both reality and illusion.