"Wrong", in this case, refers to a union that results in offspring with sub-optimal immune systems.
Apparently, this level of partner selection is influenced by pheromones. Chemical contraceptives disrupt this process.
From the article:
The pill is thought to disrupt an instinctive mechanism that brings together people with complementary genes and immune systems.
By passing on a wide-ranging set of immune-system genes, they increase their chances of having a healthy child who is not vulnerable to infection. Couples with different genes are also less likely to experience fertility problems or miscarriages.
Women are naturally attracted to men with immune-system genes different from their own because of their smell, experts believe.
The major histocompatability complex (MHC) cluster of genes, which helps to build proteins involved in the body’s immune response, also influences smell signals called pheromones.
Although pheromones may be almost unnoticeable at a conscious level, they can exert a potent effect.