Even today, some men and women seem biologically incompatible. The sperm can't penetrate the egg or, if one does, the foetus won't carry to term. Some women have had half a dozen miscarriages with their partner then, after splitting up and meeting a new partner, they have a successful pregnancy at the first attempt.
On to the scenario. The Chinese Bioweapons Laboratory in Wuhan Market 'accidentally' releases another virus. The initial impact is worse than Covid or Spanish Flu, but civilisation just about manages to hang on. Then the unintended consequence is discovered - biological incompatibility is massively increased. For every woman, there are very few men compatible enough to get her pregnant.
Might it be possible that if a woman becomes pregnant after a gang rape, she's desperate enough to try to track down the father so she can have further offspring by him?
AJ