The teenager identified only as Girl D cried during her testimony at the Old Bailey, The Independent reported. She said she thought she had a love relationship with Mohammed Karrar, 38.
She said she had sex with Karrar, both at the house in Oxford where she lived with her parents and at his house. Sometimes Karrar told her to have sex with other men, she said.
Karrar and eight other men, including his brother, are on trial.
Girl D described being driven to a house in Reading where a man performed the abortion. But she said she continued to believe even after the misery and pain she endured that she and Karrar loved each other.
"I thought I loved him and whatever girls think when they get with someone: it will work out, stick with him through hard times and good times," she said.
Girl D said she was vulnerable because her parents are deaf, and she was, in some ways, their caretaker.
While some of the group's alleged victims are younger than Girl D, she was the one investigators say was recruited at the youngest age. Thames Valley Police arrested the defendants last year.