Before we talk about George as protagonist, we should definitely note that there's a strong argument for Martha as the play's main character. The protagonist is defined as the character who drives the action of the play. Up until the final act, Martha is the one that does this. She's the one adding fuel to the fire by revealing George's most painful failures to the guests. The tensions of the play increase as her attacks become more and more vicious.
In the end, though, most scholars seem to view George as the protagonist of Who's Afraid of Virginia Woolf? This is mainly because he performs the final climactic action. The play's tensions come to a head, when he "kills" his and Martha's imaginary son. He attacked Martha the whole time, but this final assault finally draws real blood. George also gets more stage time, which may help the audience identify with him a bit more than his abusive wife.