Listen here, you little **** .
"Gender" is psychological.
"Sex" is physiological.
"Sexuality" is what you're attracted to, between the two sexes and genders.
There's a reason they're different words. They're different words because "gender" is used in psychology, sociology, and (non-English) languages, and the term was developed for such uses. "Sex", on the other hand, is used in pure sciences such as medicine and genetics. "Sexuality" is simple enough, and is usually used by people describing themselves.
That's why surveys ask for your gender, hospital admission paperwork asks for your sex, and people looking for hookups are doing so based on their sexuality.

I hate having to explain this to those who don't actually do any sort of research on the differences in definition between similar terms before they pass on something they don't know about.
I'm saying this as someone who thinks the concept of "gender-fluidity" is ******** . The concept of gender is a psychological construct. If you consider yourself gender-fluid, you have a poorly developed sense of self, as your definition of yourself is in flux. You identify as either the gender coinciding with your sex, or the gender that coincides with the opposite sex.
Sex, on the other hand, isn't what you define as. It is defined at birth, and is unable to be changed, despite what some people may say, as turning your dick inside out doesn't give you a vagina, and you can't change your DNA.
Finally, there's sexuality, which falls into one of three categories: heterosexual (where you're attracted to the opposite sex and/or gender to yours), homosexual (where you're liable to post on /fit/ attracted to the same sex and/or gender as yours), and bisexual (where you're attracted to both the same and the opposite sex and/or gender).

tl;dr: OP is wrong, but nowhere near as wrong as tumblr, who insists gender and sexuality are spectrums, which is just an attempt to normalize their ******* fetishes and/or give themselves "special snowflake" status.