Frankly, it is none of your business what the sex or gender of a user is. I understand that sometimes there is money to be made by collecting this information but it is also alienating and just plain irrelevant (and I think there is also money to be made in recognizing that people can be fluid.) You can give your users an option to provide you with these details but guessing/requiring is not a good practice. On a sid…
But what if you have a whole bunch of data and want to do some aggregate statistics? "Do women use our product?" is a perfectly reasonable question to ask yourself. You don't need it to be exact, and it's certainly not reasonable to ask every user. So you use some heuristics and you get some useful data.