SAG stands for "SAG-AFTRA," which refers to the Screen Actors Guild - American Federation of Television and Radio Artists. In the context of porn video tags, this term may refer to a scene or movie featuring actors who are members of the SAG-AFTRA union, known for their professionalism in the adult film industry.
These scenes often have higher production values and can be a way to indicate a more high-end or professionally produced pornographic content. They may also include popular or well-known performers as they tend to be more prevalent in SAG-AFTRA productions.