Communities exerted pressure on people to form pair-bonds in places such as Europe; in China, society "demanded people get married before having a sexual relationship" and many societies found that some formally recognized bond between a man and a woman was the best way of rearing and educating children as well as helping to avoid conflicts and misunderstandings regarding competition for mates.
Generally, during much of recorded history of humans in civilization, and into the Middle Ages in Europe, weddings were seen as business arrangements between families, while romance was something that happened outside of marriage discreetly, such as covert meetings.
However, one particularity of the human species is that pair bonds are often formed without necessarily having the intention of reproduction.
In modern times, emphasis on the institution of marriage, generally described as a male-female bond, has obscured pair bonds formed by same-sex and transsexual couples, and that many heterosexual couples also bond for life without offspring, or that often pairs that do have offspring separate.
Women eventually won the right to vote in many countries and own property and receive equal treatment by the law, and these changes had profound impacts on the relationships between men and women. In many societies, individuals could decide—on their own—whether they should marry, whom they should marry, and when they should marry.
A few centuries ago, dating was sometimes described as a "courtship ritual where young women entertained gentleman callers, usually in the home, under the watchful eye of a chaperone," but increasingly, in many Western countries, it became a self-initiated activity with two young people going out as a couple in public together.
The protocols and practices of dating, and the terms used to describe it, vary considerably from country to country and over time.
While the term has several meanings, the most frequent usage refers to two or more people exploring whether they are romantically and/or sexually compatible by participating in dates with the other.
While pair-bonds of varying forms were recognized by most societies as acceptable social arrangements, marriage was reserved for heterosexual pairings and had a transactional nature, where wives were in many cases a form of property being exchanged between father and husband, and who would have to serve the function of reproduction.
The person who is further along in coming out will invariably be dissatisfied with the more closeted partner. I've seen so many clients who have been in a relationship for a year or more who wish they'd established this habit early on so that it didn't feel like it was some sort of rejection of their partner later in the game.
I've seen it hundreds of times and it's always the same story, though each couple hopes theirs will result in a different outcome, with the closeted partner assuring the out partner that she will eventually come out but that she's just more of a 'private' person, and the out partner swearing that she doesn't even care if her partner comes out -- the important thing is that they're together. Time alone is one of the most important elements needed for sustaining a relationship.
From about 1700 a worldwide movement perhaps described as the "empowerment of the individual" took hold, leading towards greater emancipation of women and equality of individuals.
Men and women became more equal politically, financially, and socially in many nations.
If you want an open relationship, clearly express it to your partner; likewise if you want a monogamous relationship.