Most actors feel uncomfortable filming raunchy sex scenes. Gucci Mane is not an actor, therefore he fell asleep.
During the filming of Spring Breakers, Gucci and his crew smoked so much weed that director Harmony Korine said "Gucci was basically catatonic." It's not that he wasn't interested in his love interest (A girl who was reportedly cast after balancing four Coke cans on her butt while walking across the room). It's just that he was exhausted! He fell asleep while his female co-star was on top of him for 45 minutes. When he woke up, he said: "I feel real nice. I feel like Mozart's on my dick." Nice, Gucci.