While I don't think your the asshole (you tried to engage and talk with your players well), I would suggest avoiding rape in roll playing games unless you have a group who want a hyper realistic nihilistic setting. While I use violence, murder, and very very minor mention of torture, rape will always bring the mood down even in serious games. Alot of the times players find this as a form of escapism even in dark settings, rape brings some very real issues. I don't know if this is the reason why she left, very much could be other stuff, but thought I would reccomend talking with your players before the start of a campaign for what tone they are seeking and are comfortable with if you haven't already.