and asari can reproduce with themselves to fulfill both roles. If asari has a male appearance or no female attributed features then they wouldn't be consider female by a lot of people. As I said, gender and sex are two different things.
There is a family of lizard on Earth, the Cnemidophorus and Aspidoscelis geneses to be specific, where every member's sex is purely female, and they reproduce the same general way as Asari do via parthenogenesis.
You are correct about sex and gender being two different things. Here is the difference:
Sex = male and female
Gender = masculine and feminine
So in essence:
Sex refers to biological differences; chromosomes, hormonal profiles, internal and external sex organs.
Gender describes the characteristics that a society or culture delineates as masculine or feminine.
So while your sex as male or female is a biological fact that is the same in any culture, what that sex means in terms of your gender role as a 'man' or a 'woman' in society can be quite different cross culturally. These 'gender roles' have an impact on the health of the individual.
In sociological terms 'gender role' refers to the characteristics and behaviours that different cultures attribute to the sexes. What it means to be a 'real man' in any culture requires male sex plus what our various cultures define as masculine characteristics and behaviours, likewise a 'real woman' needs female sex and feminine characteristics.
By this criteria, the sex of all Asari are female. Their gender is female as well.