This is a trap. Let's just get all totally sexist here for a minute, and talk about root biological narratives that defined our species for a hundred thousand years before the modern era. Men could impregnate as many women as they could convince to have sex with them, and successful genetic continuation benefited from that strategy. Women could only bear one child at a time and it was an enormous investment of their time and resources to do so, so they naturally wanted as much support as possible from a father to ensure that child survived. Marriage is a social contract that evolved as a means of convincing men to stick around after the sex, to help contribute resources to successfully raising a child, and of offering women the security of knowing he would stick around, so she would be willing to invest in a pregnancy. It was supposed to benefit both parties in different ways, but from a purely biological standpoint it offers little additional benefit to men.
The biologist in me has to jump in here.
When we look at monogamy versus polygamy, we see monogamy in species where both parents are needed to successfully raise offspring. We don't see it where one parent (usually but not always the female) can do it all by her/his self.
We see monogamy more in birds than mammals, because both parents can feed the offspring (they are not mammals) and when babies are hatched helpless (think robins, raptors) it takes two parents to feed them. We also see the female birds in monogamous species put the courting male to a lot of effort before she accepts him - she wants to get good genes and a mate who will stick around. Females of species who will raise their babies by themselves also test the males, just for other characteristics. In some polygamous species very few males will do the mating, most males won't get to do any at all. Monogamy is actually a good deal for human men, they may only have one (or a few) women, but they are not in a situation where powerful men have huge harems and most young men will be totally excluded. Read the bible for examples. Not to mention Genghis Khan's genetic legacy, his Y chromosome is all over the place where his empire lasted longest.
Monogamy is less common in mammals becasue in most cases the mother can feed the baby(ies) all by herself. Males may have various social roles but being Daddy is not one of them. We do see monogamy where it takes both parents - wolf packs and meerkat colonies (alpha female is the only one to have babies), beavers (generally takes 2 to start a beaver colony because of the work of building the dam, and in a larger colony the extra beavers are last year's babies), for example.
Obviously humans are not innately monogamous, or polygamy would not be a social issue ever. However in most human societies a woman will not have a lot of success rearing her children alone, especially if she has lots of them (i.e. most of human history). Since, when we really get down to basics, the point of sex is to have offspring (speaking biologically here, not socially) any social setup that improves the success of his offspring will be of long-term benefit to a man. If he is a lousy father his offspring will not do well, and his genes won't get passed on as much as if he had been a good father. Of course there are lots of other hazards - when a woman might bear 15 children and have 1 or 2 survive, how good a father he was may matter less. Unless his behaviour is a contributing factor to the children's deaths. Part of the problem is that this is not true for our close relatives the chimpanzees and bonobos. It is true for some other primates, so it is not just us that form monogamous bonds, but we are starting from scratch, so to speak.
There are lots of ideas floating around as to why we went this route, but since they are all "just-so" stories it is hard to say how important various factors were.
OP has to deal with life today, not our evolutionary past, so back to our regularly scheduled programming.
Given what I wrote, with reliable birth control there is no biological reason to get married until children are planned. At that point marriage is important because it gives legal protection
to the children. Which in case of a divorce means child support and possible alimony (if a parent, usually the woman. has never developed much of a job history because s/he was a SAHP, that is a liability
to the children unless the other parent continues to provide some support to the stay at home parent). If we stop looking at marriage laws as something to protect the participants, and look at them as protection for the adults but more importantly for the children, they make more sense.
Back in the day we got married becasue once we started having sex we were going to be having children (barring infertility). Now a couple can get their relationship on a good footing (including the sex part) before they start thinking marriage and children.