The person is sexually aroused by dirt, mud, or filth. I think my old dog was a Mysophiliac... :P

mysophilia(adjective, mysophilic): 1. a paraphilia of the fetishistic/talismanic type in which sexuoerotic arousal and facilitation or attainment of orgasm are responsive to, and contingent oneself-degradation and self-defilement by smelling, chewing or otherwise utilizing sweaty or soiled clothing or articles of menstrual hygiene [from Greek, mysos, uncleanness + -philia] 2. the condition in which a person is dependent on something soiled or filthy, for example, sweaty underwear or used menstrual pads, in order to obtain erotic arousal and facilitate or achieve orgasm. There is overlap between mysophilia, coprophilia, and urophilia. There is no technical term for the reciprocal paraphilic condition, namely [of supplying] {being the provider of} the mysophilic materials.

Dictionary of Sexology Project: Main Index