Ariel is male. You can find out more about him by visiting the eNotes link below. Here is an excerpt:
He is a spirit of the air. In I.ii.250-93, we learn that Ariel was once the servant of Sycorax, a wicked sorceress who had imprisoned the spirit in a "cloven pine" for refusing to fulfill her "earthy and abhorr'd commands" (I.ii.277,273). Ariel remained trapped inside the tree for twelve years, crying out in pain, until Prospero arrived on the island, released him, and bound the airy spirit to his service.
As far as I know, Ariel is genderless, he is merely a sprite of the air. According to most scripts, Prospero rarely calls Ariel a he or she. The people who I know to think Ariel as a man also think he is a bit homosexual.