Usually refers to ass of female person with questionable morale, better known as whore or slut. Sometimes sperm garage can refers to that whore person itself.
*Situation one
Dude1 : "Hey i got some extra money from drug trafficing this weekend, wanna go over and spend it on sperm garages with me?"
Dude2: "Sure dad, i havent visited prostitutes since last week!"
*Situation 2
Dude1: " Hey bitch, wanna come over tonight? I want to fill that sperm garage of yours!"
Whore: "Sure baby, i cant wait for you to cum in my ass"