Love breedings like that. Don’t go for it usually but had some anon face down ass up fucks happen and they had huge dicks. Totally huge loads to boot, definitely has an appeal.
Either the top used the pen to update a "load count" that the bottom kept as hash marks on his ass, or he used the pen to mark the bottom's asshole as a fuckhole with arrows and/or a bullseye showing where to insert dick, dump load.