The reality is probably more similar to the reality during most of human history. It just isn't usually hidden any more.
Nowadays, if you decide to marry someone, anyone, you just do it. The expense is what you make it. So are the ceremony and stipulations. This means that a lot of people are married who probably wouldn't be under more strict guidelines.
In most cases you don't need anyone's permission. Multiple-partner marriages that are adding a spouse being one exception. It must be by unanimous consent, of course. But, if no consent can be had, nothing prevents the unhappy members from leaving to find more suitable arrangements.
Sometimes, because of the age of one of the participants or other circumstances, family members may have objections. If no solution can be worked out privately, arbitors can be hired. If no solution comes from that, elopement seems to be pretty standard. Some things don't change much.
You can have any sort of ceremony you and your Chosen wish to have- or none at all. You can try for "'til death do us part", or have a set expiration date. Divorce is seen as a business arrangement where property is divided and obligations are fulfilled. It usually still hurts, but the hurt isn't magnified by stupid meddlers.
On the whole, I'd say relationships are happier than they have historically been. There are no coercive barriers to entering or leaving a romantic relationship. Since the myth of "one size fits all" has been tossed in the scrap heap of failed delusions, people are free to find the situation that suits them best, even if they are the only people on the planets who have ever tried a particular arrangement.
Someday, maybe even I can find a relationship that adds more to my life than it drains. Hope springs eternal.