The capitalist definition of family follows a role model that comprises the following elements: Father, mother and children. With the first element being the title role, a friend institituition of the capitalist family called "patriachy". These points are easily exemplified by looking at almost every family around.
The biological function of a family is to reproduce and increase the odds of genetic perpetuation. The social function a family is to create individuals that share the same values of their parenting individuals, efectively recreating them, if you will. ["One of the primary functions of the family is to produce and reproduce persons—biologically and socially."
We now skip fact and dive into theory.
Family is an invention. At least family as we know it, the aforementioned capitalist families, which, although present in more than one way, was cleverly designed to maintain order and enshrine bourgeois values. And this model, widely present on western societies, is rarely criticized. The ordinary individual fraternizes and even blindly agrees that there's no alternative to this role model. This can be observed by how an appointment with a conservative psychologist goes; the professional will base some of his or her treatment based on questions relating the subject's familiar structure. If the patient's family doesn't match the "natural", this will add up to his/her emotional unbalance charge. That is, according to the psychologist, of course.
The restraining effects of the capitalist family wouldn't be too damaging if it was just excluding by nature, but it's also the model that has seen almost universal acceptance by now, effectively shunning any possible alternative. This leads to individuals whom don't compact with this order to be treated as social unfits, since that's what they're supposed to do. The model won't bent for individuals, but the inverse is, and should always be, true, according to the model.
Extrapolating, since any model that escapes the capitalist definition of family is condemned and instantly rejected or frowned on, family would be the core pillar that makes elements of society (and, if we truly are being realistic here, the vast majority of society
) disapprove and fight against other family variants, such as the differences that results from gay marriage, or children raised exclusively by one of the parents.
The thread is, as always, open for discussion and new thoughts.