Javier Chica was awarded two yellow cards in the space of 27 minutes before an offside flag denied the hosts an opening goal.
The best chance of the opening exchanges fell to Juanfran, who fired just over from 30 yards out for the visitors.
Betis's best moment came 20 minutes in, when Chuli skilfully brought down the ball and cut inside but his effort was easily dealt with by the goalkeeper.
Then after 30 minutes, Chica was shown a second yellow card after pulling down Baba Diawara.
Betis continued to push forward despite their one-man disadvantage and had what looked like a perfectly good goal ruled out for offside when Jorge Molina raced clear and slotted the ball home shortly before half time.