"I don't know many rules to live by,' he'd said. 'But here's one. It's simple. Don't put anything unnecessary into yourself. No poisons or chemicals, no fumes or smoke or alcohol, no sharp objects, no inessential needles--drug or tattoo--and...no inessential penises either.'
'Inessential penises?' Karou had repeated, delighted with the phrase in spite of her grief. 'Is there any such thing as an essential one?'

'When an essential one comes along, you'll know,' he'd replied." 
― Laini Taylor, Daughter of Smoke & Bone