Looking for 90s book title - Pandemic cure causes sterility and children sex change. Plot: There is a pandemic. The cure causes selective sterility mother's must find fathers with different blood types to conceive a child. At puberty, males change to females or females change to males due to menstrual cycles.
This might be "The Breeds of Man" by F.M. Busby. With publication date of 1988, it more or less fits with your memory of a 90s book.
Scientists find a cure for AIDS, but a consequence is that women become infertile to a second pregnancy to men with the same blood type as the first. To fix this problem, the scientists create a second type of human, the Mark II, described on the cover blurb as "a remarkably adaptable breed of man able to survive when normal humans cannot". However, it is discovered that when they hit puberty the Mark IIs cycle between genders every month.