>>74454658I actually had it explained to me before by someone who drew smut.
tl;dr it's just for contrast when drawing without color because it allows artists to be lazy due to not needing as much detail for the viewer to figure out what the fuck is going on, and if the girl is tanned then the guy will be pale instead.
It's why you see it a lot in low-quality doujins with those malformed ojisans that don't have a face or hair or defining muscles most of the time. Essentially a step above just having floating hands and a penis (no balls).
