‘Sex’ or ‘gender’ on apps?

I'm a little bit hesitant to use the word Sex in the design of my website, my other choice is to use Gender but based on this discussion it states that grammar-wise I need to use Sex for it refers specifically to either male or female.

My another choice is to just let the user choose whether they are male of female without indicating Sex or Gender just like this

What should I do if my users ranging from various races?