The porn video tag "7-sirens" refers to a specific type of fetish or fantasy theme. In this case, the number "7" is likely referring to the Sirens from Greek mythology. These mythological creatures are depicted as beautiful women with the upper body of a bird. They were known for their enchanting songs and voices that would lure sailors and travelers to their doom.
In the context of pornography, this tag could be used to describe content involving seductive women singing or using their voices to enthrall others, leading to sexual activities. It may also be related to a particular scene or series featuring seven such Siren-like characters engaging in various sexual acts or scenarios.
Note: This description assumes that the term "7-sirens" is a reference to the mythological creatures and not an interpretation of the actual meaning in another language or context.