I suggest to refine this statement, because as written, it posits
the biological and social parts of reproduction as equals.
This is slightly misleading since the basic biological facts of reproduction have not been modifiable so far in human history. Reproduction has always needed a fertile man and a fertile women having sexual intercourse. (Note this may change in the future with technological advancement, e.g. cloning and artificial wombs.) In contrast, the social aspects of reproduction have been varying a great deal on the surface.
I'd rather say that the social rules, rituals, taboos, institutions, all emerge and stabilise in order to give structure to and make predictable the underlying biological facts of reproduction. Note that
reproduction is humanity's the single most important task.
Giving predictable, teachable and learnable social structure to reproduction has been all the more important before humanity started worked out how reproduction works and began to be able to control it at will (genetics, contraception etc). It is not surprising that there is a great deal of surface variety of social overgrowth on top of the basal biology, since genetics is rather complicated (and not fully understood as of June 2019, e.g. epigenetics) and has a lot of randomness built in. Moreover, reproduction, being the single most important task of humanity, intersects with all manner of other social institutions. This has encouraged the ad-hoc theorising, and overfitting on anecdotes that became enshrined as social institutions.
Summary: the social aspects of reproduction have been humanity's attempts to understand, manage and control reproduction, and are a reaction to the biological complexities of reproduction.
This is slightly misleading since the basic biological facts of reproduction have not been modifiable so far in human history. Reproduction has always needed a fertile man and a fertile women having sexual intercourse. (Note this may change in the future with technological advancement, e.g. cloning and artificial wombs.) In contrast, the social aspects of reproduction have been varying a great deal on the surface.
I'd rather say that the social rules, rituals, taboos, institutions, all emerge and stabilise in order to give structure to and make predictable the underlying biological facts of reproduction. Note that reproduction is humanity's the single most important task.
Giving predictable, teachable and learnable social structure to reproduction has been all the more important before humanity started worked out how reproduction works and began to be able to control it at will (genetics, contraception etc). It is not surprising that there is a great deal of surface variety of social overgrowth on top of the basal biology, since genetics is rather complicated (and not fully understood as of June 2019, e.g. epigenetics) and has a lot of randomness built in. Moreover, reproduction, being the single most important task of humanity, intersects with all manner of other social institutions. This has encouraged the ad-hoc theorising, and overfitting on anecdotes that became enshrined as social institutions.
Summary: the social aspects of reproduction have been humanity's attempts to understand, manage and control reproduction, and are a reaction to the biological complexities of reproduction.