I get off listening to my parents have sex

Until recently I was preorgasmic, no matter how much I spanked the monkey I never managed to pleasure myself.
A few weeks ago my room was being decorated so I moved into my sisters room with her, her room is adjacent to our parents room.
That night I decided to try and blow my load again. My sister was lay next to me asleep and breathing heavily her hand was just touching my back. I started to tommy tank and I began to hear my parents groans of pleasure. I don't know why but this time I managed to release the silver milk. I don;t know if it was my sister or my parents but I havent orgyed since. I can't unless I recreate the situation!
Is it normal??