So I'm not clear on what the issue would be if, hypothetically, your girlfriend "liked it." Are you fine with her having had partners besides you? Is it the fact of her having a dark side that might enjoy or resonate with trauma that bothers you?

Unless you're her first (not counting that teacher), she's probably enjoyed sex with other people in the past.

However, I agree that there is slim likelihood of her having "liked it" in the common sense. Others have provided good explanations of why she would have gone back to him.

Like most of us, she is probably a complicated person with a dark side that isn't particularly pretty and with needs and wants that aren't fit for polite company.

I would really recommend talking to your therapist about this.