While having anal sex you pull out your dick and the receiver flexes their anus muscles, (the ones used to shit), which causes the rectum to partially turn inside out. Its very red in color and about the size of a raspberry, hence the name.
Bro 1: Hey dude, how was fucking your girl last night?
Bro 2: Bro, i was fucking her in the ass and i pulled out and she had the biggest raspberry!!
Bro 1: No way! So what did you do?
Bro 2: I made her keep it out and i came all over it!