Also known as: ニプルファック, 乳首姦, ニップルファック, 乳头奸
When an object, finger, Penis, Tentacles or object is inserted inside one's Nipples, thus the nipple being penetrated.
Tag with Nipple Object Insertion if it's an inanimate object. If it's an Injection use Nipple Injection instead.
If it's the nipple penetrating something else tag with Reverse Nipple Penetration instead.