Maybe it’s like that idiom, the seme “wears the pants” because of authority, showing that they are the person in the relationship who decides things, have more control, etc. As for the uke, they are stripped down to nothing, exposed, in a vulnerable state. Not sure~ but I interpet it as a power assertion sort of thing. Also, by keeping their pants on, the seme could decide when or whether the uke would receive their dick, lol~
Stereotype, cliche, unrealistic, unreasonable, power Imbalance, projecting the top as manly, stoic, unemotional makes it hot apparently and bottom as the feminine and hence making them look more vulnerable, allowing the female readers transpose themselves as the bottom. However it's completely senseless and disregard any natural male personality and murder reality.
im just wondering why most in yaoi sex scenes the seme's are only half naked they kipping the pants or shorts or boxers, while having sex. on the other hand almost all the uke's are totally naked?. aww why why??? tell me??