> It's not about a couple (or group) and their feelings towards one another; it's about the formation of a family and the production of a new generation.

If you look at the actual legal structure of marriage outside of who is permitted into it, that's very obviously not the case for the civil institution of marriage -- virtually all of the legal effects are directed toward the creation of a legal relationship of mutual support between the partners. Even the pieces that relate to children (e.g., the presumption of paternity) directly serve the purpose of reinforcing the mutual support relationship between the partners.

One could argue that one of the (many) social functions served by this mutual support arrangements which justifies having a prepackaged, publicy-recognized set of obligations, permissions, and legal privileges for mutual support is the creation of a superior environment within which to raise children, but there are other public functions served, and n any case an environment for raising children doesn't require that children be produced by the people raising them.