Marriage is a personal and cultural institution (and traditionally a state institution as well) whereby two people have an existing romantic relationship recognised by their families, community, and possibly the state. Liberal marriage is such a marriage within the context of liberal individuals, possibly a liberal culture, and possibly a liberal state. Depending on the cultural and legal institutions surrounding the relationship, it may exist in all three forms in ceremonies and practice, or as many individuals and cultures have found themselves in legal systems not organised around their values and traditions, it may be distinct (some subcultures routinely have separate religious and civil marriage ceremonies and may decide to forego one or the other ceremony or marriage for various reasons - in rare circumstances someone may be in a religious marriage with someone and a secular marriage with someone else). Marriage, being tied to love, posession, and social/behavioural cues ranging from the fully modern to the primal, smooths the borders of the relationships it represents, both between the members and outsiders.
A liberal marriage takes place between two people who are in majority, primarily for reasons of love (and never caused by duress, contract, arranged by families, or otherwise selected, pressured, or bound by outside circumstances). Marriages for convenience, wealth or power, nobility, preservation of race/family/other identity, or other primarily non-love reasons are not liberal marriages, and while they may be recognised by liberal states, they are not to be encouraged. Liberal marriages may have any permutation of the two genders, any permutation of castes (which are themselves not recognised in liberal societies), ethnicities, or original cultures. A liberal marriage is a type of cultural marriage - many of these specifics may have parallel institution in law.
Marriage, as it is external recognition of an existing relationship between people, has a number of cultural defaults that society expects unless there are indications that both parties in the relationship make clear their relationship is different. The defaults, in this case specific to the liberal context, help clarify to others the boundaries, needs, and expectations of the couple with each other and others - all societies have different defaults for these reasons, aiming to protect the relationship itself, those outside it from the two in it (particularly as boundary-crossing can easily lead to violence), and in some cases those in it from the other (e.g. if someone is not faithful in a relationship where faith is expected, the default expectation of monogamy protects the interests of the cheated by making it possible for others to intervene). The defaults may be modified in particular marriage to make them more or less stringent (however, society may disapprove and in some cases forbid (an easy example would be marital rape) some forms of stringency that conflict with liberalism).
The ceremony of marriage is not specified. Secular liberals should not have a marriage take place in a religious institution/building or with a religious ceremony (a religious building may be acceptable if the religiosity of the place is not the reason for the location, and secularised variants on religious ceremonies may be acceptable to the extent that they typify culture), although religious members (who nontheless are culturally secular liberal) of secular liberal society may have a religious marriage ceremony. At the beginning of the marriage, a couple should have a reasonable idea of any differences from the liberal marriage defaults that their relationship takes - they may devise, if they so desire and particularly if culture creates it, means to express any of these differences to society at large (or particular subgroups).
A liberal marriage may be terminated by either partner by providing notice to the other partner (this should be instituted as a legal norm). This notice should be provided in person if possible (in cases of mental incapacity or great distance, this might not be feasable). It is considered a great wrong to leave a partner for a prospective partner one has met during a marriage, and society should shun those that do so. After divorce, a period of waiting (at least three months) is expected of both partners before they will seek new relationships or try to meet new prospective partners (this is a guard tradition against violation of the above great wrong).