This happened when my friend was having a sleepover at my house. My mom brought a date home that same night and ended up having sex with him. Me and my friend could both hear it because her bedroom was right beside mine.
I’ll never forget the quiet tension in the room when we went to bed because we both had sort of a feeling something was going to happen because she had been drinking a bit and she was acting kind of silly. But when I heard her go into her bedroom with him and close the door, then all the giggly kissing and stuff started up, I was thinking, “Holy shit, she’s actually going to do it!” And I got an instant erection and a surge of butterflies like I’d never felt before, and that accompanying hot pressure feeling you get in your temples and face when you’re that excited. That’s such a flood of weird conflicting emotions at that age