>>271685Contrast and degradation/humiliation fetish: The bigger the gap between the male character's appearance (gross, old, sweaty, obese, balding, creepy) and the female character's (usually idealized as young, beautiful, pure, innocent), the stronger the sense of "fall" or corruption feels. A hot guy seducing someone isn't as "degrading" for this kink; an repulsive man "winning" or forcing the situation amplifies the taboo/shame element that some viewers find arousing.
Power fantasy via relatability or wish-fulfillment for some viewers: The "ugly bastard" is often portrayed as average-or-below-average in looks/status/power at first glance, yet he still gets the attractive woman (through coercion, money, position, persistence, or sheer endowment). This can serve as a self-insert boost for viewers who don't see themselves as attractive ("If even that guy can get her, maybe there's hope"). It's the opposite of the perfect handsome protagonist in vanilla romance.
Cultural coding of "depraved/perverted" characters: In Japan, where thinness is the strong norm and obesity is rare and heavily stigmatized, fatness is an easy visual shorthand for "disgusting degenerate," "gluttonous," "low self-control," or "creepy old salaryman/pervert." It's similar to how Western media sometimes uses similar traits for villains or comic relief creeps, but dialed up in pornographic contexts.
Niche fetish demand: There's a dedicated audience for this exact contrast (ugly/fat guy + hot girl), so artists and doujin circles keep producing it because it sells. The same pattern appears in some live-action JAV (Japanese adult video) casting, where "normal/ugly" older men are deliberately chosen for certain genres.