In her bedroom

A girlfriend and boyfriend walked into the girlfriend's house and the girlfriend said to her mum "Mum, me and my boyfriend are going up to my room" and the mum says "Okay honey, you kids have fun".

When they are in the room, the mum hears "Baby baby baby oh!"

The mum walks to the door and ask "What the hell is going on?"

The girl says "Mum were just having sex".

And the mum says "Oh thank god

Spoiler:

I thought you guys were listening to Justin Bieber".