Were it me? I see your concern, but I don't think it's a thing about what her ex did that was sexy. Rather, it's just something she finds sexy that many people do...including her ex(s).

I wouldn't worry about it too much. I don't think she's thinking OF the ex(s) while she's with you. I just think she's missing an aspect of her sex life that she wants you to be a part of. That's how I'm seeing it, anyway.