‘La cama’ is femenine. ‘El sofá’ is masculine.

Limpié el sofá y me dormí en él.

I cleaned the sofa and I slept on it (him)

You are right, that’s what it says, gramatical gender is quite interesting isn’t it? The same goes for other languages like French and German. Idk why you’re getting downvoted, this is a genuine good question.