The reason for adding a sex fact probably has more reasons than these, but these are the most obvious:
1) To clarify which sex for bi-sexual names like: Alex, Leslie and so on.
2) To clarify sex where the name is unreadable to try to assist what the handwriting really says.
3) FTM uses the sex to enter the surname of people that you add to a family. People are entered with the surname of their father - which is established by the sex of the couple who had the children. The entered sex of individuals determines whether they become the "father" or "mother" in a coupling.
For these reasons, and particularly Number 3, FTM / Ancestry will NEVER remove sex from adding to the fact, where known, from various records.
Sex is currently added for US Censuses, for 1850 and later, because sex is in the census. This is the ONE fact that you do not have a choice on when merging. In this context, it is already "automatic", although "standard" would be a more accurate description. This will continue and there is no chance that they will change on this - nor would we want them to. Other censuses (states and other countries) will depend on whether sex is in the original record.