Depends... biblically speaking it is a bit fuzzy. In general, sex was the method of consummating a marriage. However, there had to be an agreement. So you could be engaged and the engagement would be culturally considered as a marriage but the difference was cohabitation.
The reality is these were cultural norms, not necessarily "thus saith YHWH", that have all been tossed out the window in this day and age.
And if you want to understand why... well the ancient model had many problems. Women's rights, for one, were almost nonexistent. Women were not treated as equals. Men basically sold their daughters. You could argue that the bride price was paid to show the husband could take care of her but in reality anything that can be used for profit will be used for profit. And while some men are honorable and love their daughters, some men will kill their daughters for denying them their "right" to sell their virginity.
So there's that.
The ancient system was supportive of arranged marriages, like I just pointed out. Falling in love wasn't always the goal. Women were even taken as spoils of war and given little time to mourn their slain families.
And then men could also marry multiple women. So what happened then? They would abandon one in order to take another. The law changed to try to prevent this so it established that you still had to take care of previous wives to the standard of living they were accustomed to.
But there's another weakness in this system.
It doesn't actually let the 2 people figure out if being together for the rest of their lives will work. There was nothing about compatibility, personality, etc. So for many reasons a wife may not have loved her husband or vice versa. Sure, she looked good but is that enough?
So this is why society changed and we started living together first and having sex first to test our compatibility over a period of time before making such a big decision that would deeply affect the children. Many people thought that children were good as long as their parents were together and therefore marriage vows were always "til death". That's not for the benefit of human beings in love. If you're in love you don't have to promise to stay with someone. You stay with them because you love them. Period. Marriages were contracts based on property rights. Property owners wanted assurance that their investments wouldn't run away.
Now we know that it is better to be happy and for children to see happy and healthy relationships. Children whose parents stayed together aren't handing out medals if that relationship lacked in love and affection.
Are we perfect now? Do we have relationships figured out now? No. But what we're doing now is better than ancient times. You just have to look at it from a human perspective.