I think it’s indeed not fear of appearing desperate, but fear of being judged based on your promiscuity. I have done quite a lot of reading into this matter, because it really peaks my interest (I consider myself a feminist, but notice these biases in myself as well, why?). There’s not a lot of (scientific) literature on the matter of so-called “slut shaming”, (or so I have found) but one of the main reasons to judge women is because they come across as being less trustworthy — women even perceive promiscuous women as being lesser friends.
Whenever a woman takes initiative, takes the dominant position in relations or communication, it seems that they are more promiscuous and more threatening to other women. Social norms of women being the ones to “follow”, are quite sturdy I guess.