As for the protection of minors, we have quite simple 18+ rating rules for photos, clearly symbolized below.
Reasons for an 18+ / adult classification
- visible genitals; beginnings of genitals (also through clothing)
- clear depictions of sexual acts
- Images of sperm (on face, clothing or objects)
- Explicit images of golden shower
- Harder SM practices and their consequences, e.g. welts and bruises