Why is sexual inferiority a turn-on? What evolutionary pressure caused the trait of inferiority to be paired with sexual arousal? The standard view of evolution through genetic competition has no explanation for this.
Perhaps a cooperative view is in order? We can theorize that sexual competition might ordinarily result in the elimination of the weaker member of a tribe, especially if the weaker member continued to attempt to compete. Eventually, injury or death might occur prior to the weaker member being able to pass his genes on. In the alternate case, a cooperative weaker member would be more likely to survive. Is it possible that through survival, there would be a greater chance that he were allowed to breed eventually, perhaps with a less desirable mate?
That would explain why these genetic traits were not eliminated thousands of years ago. It would also explain why this fetish is also typically accompanied by the fetish of subservience and cooperation with the stronger male.
It should be noted, however, that unfortunately for the male with this fetish, there is no reasonable expectation that a female would be aroused by the sexual inferiority of a male. For a mutually-supportive couple to exist, she would have to be aroused by the idea of sexual dominance over a physically stronger male. There are a number of reasons why that trait would be preserved by evolution.