This is not a debate. You may freely call it whatever you want. You however may not debate, express disagreement, or argue against what the site is going to call it. (TOS 2) OOannie has been banned for 7 days for forgetting this in her response after one of her posts which crossed the lines was deleted.
I used the word gender because when they hear sex anything many people automatically think sexual intercourse, and once they get that picture in their minds all reason goes out of their brains. "I don't want me or my kids to have sex with someone who used to be a man. We gotta fight this..."
I can't tell you how long it took to convince someone who I see as a brother the simple idea that this was about gender, not sex; it was about making my body reflect the gender I already am. It took him months of soul searching to get past the idea of a person he knew as male having sex as female; to understand my point that my transitioning was about gender and not sex, and he had a trans family member.
That is why we use gender because, transition isn't about sex.