I'm confused as to where you got the idea that having a sexuality means you're attracted to every single person of a certain sex. It just means that you can be attracted to people of that sex. I'm bisexual but I have standards, I'm not attracted to every person on earth. Sexuality categorization is useful to quickly contextualize discussions about sex and relationships in this world where heterosexual is assumed to be the default unless explicitly specified otherwise. Doing away with sexuality would only add confusion and wasted time. For example: looking for advice in gay dating but receiving advice about straight dating (that doesn't apply to gay dating because the cultures and dynamics are very different) when there's no terminology to tell them apart.