Bacteria and Viruses make people sick and cause death. These 1 cell and 1 sex organism multiply much much faster than Animals/Humans and evolve much much faster.
Sex and Gene mixing makes the offspring better prepared for the Germs and Illnesses that already exist, if all people had the same genes a single strain of a Illness would wipeout the entire human population.
second point - All should have the same genes so its "fair", in that case they should have the same Education and same parents(Maybe raised in camps like the Hitler youth??) Maybe we should all wear the same cloths so its all "fair". I dont buy in to this bull shit, we should Love our diversity and uniqueness, instead of looking for ways to become a grey goo