what is the meaning of this sentence?

as written, it seems to mean that the pornography use of the person's previous partners has no value for predicting the self-esteem or body appreciation of the individual.  That is likely not what they want it to mean, they want it to mean that women have more negative thoughts about themselves (lower self-esteem, less favorable body image) when they have previous partners who had higher levels of pornography use.

negatively predicted means did not predict, in my understanding of the phrase. negative ought to apply to the outcome rather than the act of predicting. negatively predicted means not predicted at all; badly predicted.  Surely that is not the intended idea.

The correlation between act and prediction might be negative, the predicted outcome might be negative, but there is nothing negative about the predicting itself if it works.Just the opposite, the prediction works, is functionally positive.

They seem to be trying to say "Women feel worse about themselves if their past partners liked to look at pornography".  Presumably, the author(s) are suggesting that women compare themselves with the women in pornography and feel their own bodies are not as good. The reference to self esteem COULD imply that the women feel they may be seen as mere sex objects by their past partners who, presumably, see porn models that way.