The porn video tag "milf-guy" refers to a scene involving a mature woman or mother (Milf, an acronym for "Mother I'd Like to Fuck") and a male participant. In this context, the term "guy" is used to describe the male character in the video. This tag indicates that the video features a situation where a younger or average-aged man is engaging in sexual activities with an older, experienced woman. These types of videos are often enjoyed by viewers who appreciate the dynamic between different age groups and experience levels in sexual encounters.