Fucking. Duh.
Women's mate selection changes when their body is flooded with synthetic progesterone. In general, a well adjusted fertile woman will seek men who have a tribal similarity to themselves, but not familial. This reduces the chance of inbreeding, even among more distantly related individuals. This attraction is based on three major factors: appearance, pheromones, and antibody signalling.
For anyone who isn't familiar, one of the main evolutionary benefits of kissing is that it allows the exchange of antibodies through saliva, which in turn allows an individual to determine the relative intersection of antibody distribution between themselves and a potential mate. A more robust cross-section of antibodies will generally improve the immune system of offspring, by giving them a greater statistical resistance to pathogenic threats.
Physical appearance signals both health and fertility, in addition to intelligence (Side note: more attractive people are more intelligent, on the whole. This was another reason why the "dumb blonde" and similar stereotypes are so effective at skewing the mate selection process and encouraging miscegenation: if individuals can be put into a false dilemma when choosing potential mates, they are more likely to make poor decisions based on a fallacious "least bad" calculus, instead of simply choosing the person that in purely instinctual terms is the most attractive.), but it also signals genetic relation: if someone looks just like one of your parents, they probably are more closely related to you than someone who simply bears a racial resemblance. Note that oxytocin, the bonding hormone, causes women to favor people who have a closer racial resemblance to themselves: or in other words, another risk from hormonal BC is that because it undermines deeper emotional bonding between women and potential mates, it erodes their natural tendency to seek mates from their own ethnic group.
Pheromones signal both social dominance and hormonal disposition, both of which are pivotal when evaluating the social status, social disposition, and overall health of mates, plus it signals nearness of relation: just like appearance, if someone smells just like your family, they're probably more nearly related to you than someone who does not.
When a woman is pregnant, she becomes more drawn toward family members - this is a way to encourage her to seek support and resources from a reliable source, to maximize the chances of carrying the child to term. Anyone who's had a wife who was pregnant will probably notice that she wants to spend more time around her parents or siblings than she otherwise would.
With a woman who's on hormonal BC, this perverts her mate-selection process (it also fucks up mate-selection for single pregnant women, but that's not as important to this discussion), by causing her to be drawn to men for whom she would otherwise have no attraction at all, as they remind her of her immediate family, particularly in terms of scent.
There is a well known phenomenon that when women who have been on BC since they started dating go off it to have kids, there are often cases where she suddenly finds her husband repulsive, or stops being capable of seeing him as a sexual object, even if they had a semi-healthy sex life beforehand. They often describe the smell of their husbands as intolerable, even if they previously found it comforting. This causes BC to have a "double-protection" effect, since women on BC are physically incapable of reproducing, but women off BC will not be attracted to their current mate. This can result in either cuckoldry, since her reproductive impulses will be stronger with no acceptable outlet, or simple divorce.
Either way, (((they))) win.