The initial plot was about genetic engineering being used to defeat disease, but something went awry that caused a massive social upheaval, where people in social settings would have a lapel pin or small badge that showed their blood type, so they could match up with someone compatible ( and not catch/cause a worse disease ).
An unknown effect of this genetic engineering was that newer children born turned into a sort-of hermaphrodite where they would physiologically switch genders under certain circumstances ( either puberty or pregnancy, I don't remember ).
The protagonist eventually adopted the surname Dos Caras ( meaning two-face or two-faced? ) and ended the story with a compatible mate.
All I remember of the antagonist was that he was a real ass, and regardless of the gender he/she was at the time, would flaunt the social custom of showing blood type in a gathering ( by wearing whatever blood type corresponding to the person he/she was interested in, and not his/her own ) just so he could 'sow his seed far and wide' to spread the genetic mutation that causes children to switch physiological genders.
The antagonist might have even died, by the end of the story, but that's one of the more fuzzier parts.