Sex is what consenting adults have with each other in the bedroom. Many times the activity is called intercourse. Gender is a description of the body type: masculine or feminine. When a person asks, what sex are you?, they could say, “What gender are you?” Male or female? Therefore when the employment form asks Sex and cousin Jethro says Yes, the employment form could ask gender M or F and Jethro should identify as being Male. Of course if he is looking to have intercourse with a woman then he could circle F that he wants to do it with a woman since he is a heterosexual male. I hope to have simplified this entire matter for those of you who may be somewhat confused by the entire narrative.
Gender is fluid because it is a language term and language has to be fluid.
Aviatrix was the feminine gender term for a female who flew air planes which has fallen into disuse as the gender neutral term "Pilot" came into common usage.
The correct word for the state of being biological man or a woman is sex. It has always been sex.
Sex is solid because it is based in biology. It is not fluid. A mammal, which is what humans are, has only two sexes, male and female. That is immutable.