There are many reasons for fixed cats marking areas of the house. One could be because of stray cats marking areas outside of the home. This causes your cat to feel they need to protect what they have. Another reason could be crystals in your cat's urine which often happens with fixed males. This is a life-threatening situation and requires a visit to your vet. Finally, if another cat brushed the furniture before you got it, this could cause your cat to mark it with their scent. The scent on you that brushes them can also be an issue. Use the process of elimination and speak with your vet for further assistance.