The porn video tag "stf" is most likely an abbreviation for the term "Straight to Fuck." It refers to scenes in which two or more people immediately transition from any other activity, such as conversation or foreplay, into sexual intercourse without any delay. This tag implies a quick transition to a high-energy, passionate sexual encounter, typically focusing on penetrative sex.
In the context of porn videos, this tag is often used for scenes that emphasize spontaneity, intensity, and the desire to skip preliminaries in favor of direct physical contact. It signifies an immediate immersion into the act, with the performers demonstrating their eagerness to engage in sexual activity.