When I was teaching in the catholic school I taught the standard three definitions of marriage
1. A civil contract between two people that has three essential elements.
The contract is legally recognised and is registered. This is "de jure" marriage.
a) both parties must be free to marry - legally old enough and not already married
b) both parties must consent to the marriage contract free from duress
c) the commitment is a solemn one, made before witnesses and binding on both parties. It confers rights and responsibilities on both partners.
2. An informal union that has the same characteristics of a "de jure" marriage to an outside observer.
Lacking a formal ceremony or any registration, the union may still be regarded as marriage. It is considered "de facto" marriage.
3. Sacramental marriage is a religious concept. It has additional layers to the secular definitions.
For Christians it means a monogamous union of a man and a woman and is, for the most part, considered to be indissoluble. This relationship, between husband and wife, is considered to be the most fundamental of all relations, including the parent/child bond because it is for life. Children grow up and leave the family home. This is a very high standard and marriages don't always last the distance which is the reason why most churches are prepared to sanction divorce.
Faiths other than Christianity have different theologies regarding marriage but if they want to have the unions recognised in law they must meet the requirements of #1 above. Catholics regard marriage as a sacrament, protestants on the whole do not and are more relaxed about remarriage after divorce.
For myself, my personal view of marriage is in line with the traditional teachings of St Paul.
That's why I've survived nearly 52 years married to the same man.