Yes, it is. I really don't know why people think it's not. I am a transgender man, my gender was once female in the womb but became male due to exposure to a large amount of testosterone, that itself proves humans start out as female, otherwise I wouldn't have been born with a brain and body mismatch. Gender and sex are NOT the same. Gender is between your ears and sex is between your legs.
Sex is not permanent, adult humans can change sex if their genes are manipulated. The genes FoxL2 and Sox9 are present in males and females, but only one of them is activated at a time. If the gene that is activate is switched off, then the ovaries or testicles will cease to function correctly. Men and women both produce estrogen and testosterone, just in different amounts. If the Sox9 gene in a male body is switched off, the testicles will function like ovaries, and vice versa, if the FoxL2 gene in a female body is switched off, the ovarian tissue will become testicular tissue.
If my gender is male but my birth sex was female, then there is something fishy going on. Unless I started out as a female, then I must have a Y chromosome but for whatever reason didn't develop male genitals.