You can appreciate something may be objectively harmful even if you didn’t ‘think’ it was at the time.
True. But I'm not sure we can apply this to sex given with consent as it involves submitting of the physical, emotional and mental person in a way that is highly individual. It involves agency and to deprive a person of agency, is seen as equally harmful. Particularly when it comes to a woman wielding her agency, within patriarchal structure it is seen as negative.
Presumably she thinks her actions are positive, and mutually beneficial for all parties involved. She is fulfilling their desires and in return is able to fulfill her objectives. Her world view fits firmly within the current hegemony of the post-modern moral framework. To have a cap put on female sexual expression is harmful to women, results in the commodification and disempowerment of women's bodies and is 'slut shaming'.
Following on logically, it is you, and people like you with your perspective that sexual expression among women must be confined within the boundaries of a conservative norm that is based in patriarchally driven, religious notions of monogamy in which women can become disfranchised, disempowered and the possessions of men and seeks to confine them within those boundaries through shame regarding their sexual agency, that is harmful. She sees herself as freeing women from this paternalistic world view. From a world view in which men have full sexual agency and women are seen as victims if they express theirs in like manner.
She is exposing herself to illness
This does not still make it more harmful than sex with lesser numbers of men in which it takes only one encounter to pick up an illness really. And sex with any stranger is a risk.
She is perpetuating a dangerous narrative that women are merely there to be used
I would counter that many women do this on a lesser scale that contributes to large net effect of women in modern society being increasingly perceived as sex objects and puts huge pressure on young people to enter sexual encounters before they may be individually emotionally and mentally mature. You could argue that no strings, freely available sex, removed from monogamy, removed from family, largely divorced from responsibility over the last few decades has already done this.
In fact, I would argue that because she is doing this within a context that is very clearly removed, defined within fantasy and so far outside the norm as to be extremely blatant, it may, in fact, cause lesser harm (if you believe it us causing that) than the former effect, which is insidious and 'cheered' on much more readily.