User blog comment:Omega natsu2/Chapter 392: You Must Not Forget, Predictions./@comment-1359322-20140711090529/@comment-7418318-20140711101735

That can be explained by denial. At first Gray didn't want to believe his father could possibly act like that, but after some time he could no longer lie to himself.

To give the obvious parallel, Luke also initially said to Vader that he can't possibly be his father, but came to accept the painful truth quite quickly.