I have been trying to find this story all over I believe it used to be on a single spark. Sango, Miroku, and Inuyasha are captured by Mukotsu the poison master. I think he drugs them with something. Sango ends up plesauring miroku and having sex with Inuyasha. Sango becomes pregnant with Inuyasha´s baby. I do remember her giving up the baby to kikyo and inuyasha.
I´m pretty sure kagome had gotten saved by sess, which is why she wasn´t around but I´m not sure.
thanks for any help you can give me.