So apparently tags like penis_in_ass and penile don't imply penis.
I think it's because you don't see the penis when it's in balls-deep and implying it would tag those images with penis.
On the other hand this means that there are a lot of posts (not sure of exact number, several hundred) that don't have penis tagged despite one being visible.
See: penile -penis. Maybe half of these have a visible penis, I didn't actually count.

TL;DR: tag penis manually if one is visible in your images please.