holding_penis is defined rather unintuitively as "A character holding/grabbing their own penis for any reason. Holding in this context refers to gripping the penis, or some other means of physically supporting it with one's hands/arms." which has lead to an incredibly large amount of mistagging. I think it would be better to move that definition to holding_own_penis and allow holding_penis to apply to any character holding any penis, or we would have to clean up the tag so it can be safely aliased to holding_own_penis.
The holding_*/*_grab system of differentiating between who is holding/grabbing what just doesn't work, because too many users don't read the tag wiki when choosing which tags to use, so all tags following this format should probably be replaced with more descriptive tags.