Socially, the gens is much more than this. It takes all forms, from traditional monogamy to triads and larger group "marriages." It can be a corporation, a partnership, or have no economic excuse whatsoever.
In the nobility, the gens usually takes the form of an extended blood family complete with lovers, concubines, assorted hangers-on, and a whole raft of others just along for the ride. In the commoners' sphere, the gens may have little to do with blood and may change its membership many times over the years. Occasionally a gens takes on a life of its own, and some have lasted all the lifetime of the Empire.
Under Imperial law, a child is declared illegitimate if his/her parents do not have a contract to provide for raising the child to adulthood -- illegitimate children cannot inherit.