Back in the early days of cable TV, when the earth was cooling and dinosaurs roamed the land, there were four major cable channels which aired feature films: HBO, Showtime, The Movie Channel, and Cinemax.
Cinemax was clearly the runt of the litter. They didn't have the market power of HBO, so the bulk of their offerings consisted of movies that were available to all four channels (i.e. movies which weren't "HBO Exclusive!" or "Showtime Exclusive!"), or movies which had been "HBO/Showtime/TMC exclusive!" in the past but were now old enough that those exclusive rights had expired, or movies which the other channels just didn't want to show.
In this latter category was a lot of softcore porn that Cinemax aired late at night. Strictly R-rated T&A stuff, nothing hardcore. Eventually, they started making their own original softcore porn movies.
This earned them the nickname "Skinemax".
So presumably "skinemax yuri" would be softcore lesbian porn aimed at a heterosexual male audience.