There is something about the smell of pee, and cats, idunno what it is
my cat LOVED coming with me to the bathroom, and would go into an - as you say - almost erotic extasis, rolling around on the floor, purring loudly, to what I can only assume to be the thrill of my smell of shit and piss
The only possible explanation I can think of is that - the cat considers you both A TEAM - you and the cat form a tiny hunting party/small family. Cats use urine to mark territory.
The cat usually never sees you pee (never sees you mark territory), so when it finally DOES get to come along, and witness the allmighty human pee, it becomes almost extatic for them - cus you pee for the sake of both. Since humans are much larger creatures, they will also piss that much more - the very ammount of piss being most impressive to a cat
Since cats are simpler creatures than humans, intellectually, all this joy just builds into a near sexual emotion, as you fill the whole room with the stench of human-piss.