Sex-ambivalent is a term that is used by asexual individuals to refer to the fact that they have mixed feelings toward sex. Sex-ambivalence refers to anyone who does not fit into the categories of sex-positive, indifferent, or repulsed. This could be for any number for reasons; common reasons being that one's feelings about sex change frequently, so that they might be positive or indifferent one day and then repulsed the next day. Their feelings about sex might also change based upon the situation. For example, someone might be normally sex-indifferent, but if they're in a relationship where their partner is sexually attracted to them, then they might feel sex-positive. The opposite might happen as well, in which someone could feel sex-positive or indifferent normally, but if their partner is sexually attracted to them, they might feel sex-repulsed.
Sex-ambivalence can also stem from internalized acephobia, which can make so the person doesn't know if they're actually sex-positive or if any desires to have sex are simply based on the fact that they feel that people should want to have sex.
Not all sex-ambivalent people are asexual; there are also some allosexual people who are ambivalent about the idea of sex.