What defense mechanism is being used by a husband who blames his sexual inadequacy on his partner's appearance?

The defense mechanism exemplified in this scenario is rationalization. This occurs when an individual provides logical or reasonable explanations for behaviors or feelings that are actually driven by irrational factors. In this case, the husband is attributing his sexual inadequacy to his partner's appearance rather than acknowledging any personal insecurities or issues that might be affecting his performance. By doing so, he is attempting to make his feelings and behaviors seem more acceptable and less threatening to his self-esteem.

Rationalization helps individuals avoid facing uncomfortable truths about themselves, which is exactly what the husband is doing. It turns a personal issue into an external one, thus protecting his ego from the distress of confronting possible inadequacies or failures.