I think it's pretty believable that Fumiko isn't just a whore from the age of 14. Devils don't just walk on the street and hand out contracts, and unauthorized contracts are illegal. I feel like she was in a vulnerable position to make that contract especially since she was a minor, and it was with the STD devil, a negative consequence of sex, not the Sex Devil or the Orgasm Devil. Also it fits with how devils prey on a human's vulnerability like Yoru offering Asa a second chance at life when she was on the verge of death, and Cicada Man was upset at his wrinkles when looking at his reflection.