Modern marriage in "Western" countries is a steady growth out of the medieval Church's Olympian effort to Christianize marriage in the early-high Middle Ages. Medieval scholars drew on two basic traditions, Roman law and "biblical," or what they understood to be ancient Israelite practices. Modern scholars have clarified some of those ideas, plus debated whether we can talk about a "Germanic" tradition of marriage separate from Roman ideas.
My colleagues /u/gingerkid1234 and /u/yodatsracist would be better up to speed on current scholarship on marriage in actual ancient Israel. Medieval scholars were mostly concerned with trying to justify/explain away the troubling polygamy business...which, in fact, modern scholars generally suggest was not that common. Referencing other ancient Near Eastern laws like the Code of Hammurabi, many have argued that rather than "700 wives and 300 concubines," the situation was more: if a husband and wife could not have children, it was assumed to be her fault, and he was allowed either to marry a second wife or to have sex with (rape?) one of his wife's slaves. The children of a man and a concubine would have equal inheritance rights with any children of the marriage. That tells us, right off, that marriage was about reproduction (social order) and inheritance. Arguably, the focus on the woman as the infertile one likewise points to the marriage bond as a perpetuation of patriarchy in the sense of male dominance: the family does not continue on if a wife has sex with and gets pregnant by another man.
We're given a few more clues about the purpose of marriage among ancient Israelites by painstaking translation work. Scholars have extracted the idea that a key component of formal marriage was bridewealth. In ancient ancient times, this may have been a payment to the bride's father. Later on, however, it was more clearly a gift to the bride as part of her dowry, which points to the idea of marriage as helping ensure social stability in this generation, not just the next. The new couple could use the dowry to establish themselves, or a woman would have some support in case her husband died.
In ancient Rome, meanwhile, marriage was first and foremost about sealing social alliances, although it came to be just as much about inheritance over time. If you've ever read Augustine's Confessiones, you might be familiar with the standard footnote or translation about his "common-law wife." That's because in Rome, "marriage" was a legal institution specifically reserved for the upper classes. Lower class people could form temporary or permanent bonds, but they could not legally marry. And why would they need to? They had no great families to tie together for political purpose; no landed estates to pass on. Roman sex life was characterized by a multitude of relations, of which "marriage" was only one (and potentially only barely qualifying as such, depending on the people involved). In these cases, for propertied men, the "wife" versus "concubines" mattered greatly, since only the sons of a legal wife could inherit property.
There's one more tradition, or rather lack of tradition, to talk about. Into the late 20th century, scholars continued to nurture a romanticized view of a pan-Germanic marriage, one that was viewed as friendlier to women than the "patriarchal, decadent, oppressive" Roman version. More recently, scholars have completely deflated basically every part of this argument, starting with the idea that there was a unified "Germanic" marriage and ending with the idea of it being a happy lovely thing for women as a rule.
The biggest problem for assessing what we might call north-of-Roman marriages, pre-evangelization/Christianization, is that our sources are almost uniformly post-Christianization. Research has picked apart the so-called "barbarian law codes" to show that they were in fact already very influenced by Roman ideas, thus 'poison fruit' as far as excavating a pre-Christian marriage among barbarian peoples.
That said, there is one Roman author who describes a little bit about some of those practices, who was not influenced by Christianity. Tacitus' Germania has a few other interpretive problems--Ruth Mazo Karras and others have pointed out that given the context of the anecdote, he is most likely picking out the things he observes that contrast with Roman practice, and doesn't make an effort to distinguish among the German peoples. Nevertheless:
Alone among the barbarians, they are content with one wife, except for a few who, not because of lust but because of their nobility, enter into several marriages. The wife does not bring the dowry to the husband, but the husband to the wife.
Whether it's bridewealth, dowry, inheritance, or extended family ties, the general picture of marriage before the Church involves economic and social stability in the current and future generations.
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