Why is it she feels she discarded her humanity, simply because she had sex with Barret?
Just taken out of context.
In the doujin:
She, Barret, and Cloud were on the run when their vehicle got hit by an explosive that severely injures Cloud and leaves him in a coma. They're captured and kept at a Shinra facility where they continually rape her, and she can't resist them as Cloud's life remains on the line. She mentally holds on with the hope that Cloud will awaken and then they can attempt an escape together. That gets shattered when she finally gets reunited with Barret. She's really happy to see him again and thinks that with them together they can do something to escape, but he attacks and rapes her as well. He had been used as a guinea pig and through all the experiments they've completely destroyed his mind. This ends up the last straw for her mind, as she finally succumbs to despair that they'll never escape from the facility. It ends with several scenes of them raping her in the bed next to Cloud, and halfway Cloud is no longer there in the neighboring bed, with a cleaned and tidied bed where he used to be.