Quite simply put sex refers to the biological orientation of the individuals reproduction organ while may refer to the mental disposition of the person as in the behavioural, mental and physical aspects of the individual. In sociology 'sex' refers to nature while 'gender' refers to nurture.
So if you say your Gender in undetermined but your Sex is actually female then you could simply mean your not that feminine. Often times the two are confused because there is very little difference between them.
Both are words which can be used for same as well as different meaning
As far as I know, there is no difference between the two. But I believe that every gender, no matter how many of them, should have sex, because it helps to maintain health, beauty and good mood. Personally, I prefer feminization, which you can read more about if you look at this site So I, regardless of what gender I am, have sex regularly because it brings me pleasure and supports my health and mood.