The recently launched "Seen at :time:" feature (also known as "read-receipt") allows the sender of a message to know when it has been visualized by the recipient.
This is not only a major privacy issue but can also be considered a security issue regarding cases of stalking or cyber-bullying.
We, as Facebook users, have the right to control what information is displayed and sent to the recipent. Also, we have the right to take our time to reply to the recipient or not reply at all, without having the pressure imposed by the "read-receipt".
Therefore, in the resemblance of other Facebook features such as location (you can choose whether to share your location or not) or content sharing (you can choose with whom to share), Facebook needs to allow the disabling or removal of this feature.