It is natural for humans to pair off.
What do you base that assumption on?
It seems pretty self-evident to me. There are any number of species that mate for life. I see no reason that couldn't be a natural facet of human nature.
Pre-contact Hawaiians did no such thing, and their society probably closely resembles ancient hunter-gatherer lifestyles. Also, polyamory is quickly becoming popular across America, as well as open relationships.
I have yet to see an open relationship partnership which has endured.
But let's explore the notion of family from another direction. I've known any number of people who never knew their birth parents but had a deep need to know who they were. They "wanted to know where they came from". That may not be important to you, and you're entitled to that feeling, but you can't begin to suggest how others should feel about something like that.
Why should children be entitled to inherit property if marriage is without meaning or value?
What does one have to do with the other? You're not making any sense here whatsoever. If someone wants to will their property to their kids, they should obviously have that right.
I'm not suggesting that you shouldn't be able to will your property to anyone. But if you do pass away without making a will and you own property, then your children have a right to inheritance per state law. If you suggest there is no value to marriage, that everyone should be able to have sex with whoever is a consenting partner, then why should you have any specific responsibility to children just because you were a sperm donor?
The need for marriage as a cultural institution is all about providing a structure for the agreement of the rights and responsibilities of families - however you choose to define what a family is. Many families are childless and that makes them no less valid than those families that do have children. It is the family structure which conveys the rights and responsibilities inherit in them. One of the big issues in the pursuit of legitimizing gay marriage is for one partner to have the right to be considered a family member to the other, which is essential in medical decision situations, spousal support, inheritance, etc.
I have no problem with anyone choosing to not marry or to live in a polyamorous manner. For that matter I do not believe in the notion of "sin". I do believe in the right to make choices, good or bad, and that everyone is responsible for the consequences of the choices they make. I do not believe that anyone is entitled to forgiveness for making a bad choice, though it's certainly OK for those affected to do so.
I feel that everyone has the right to choose their own path in life and that includes accepting or rejecting contemporary social norms within the bounds of the law.
Personally, I think the Hawaiians had the right idea all along: no marriage, no permanent pairings, people can have sex with whoever they want, and the village takes care of the kids collectively.
In a hunter-gatherer culture, there is no real concept of property, so this is a viable social model. I don't see as viable in our technical industrial culture.
You may see polyamory as a viable life model and your partner may agree, but my wife sure wouldn't. I always have the choice to make for myself and as such, have continued to choose for myself to stay in a traditional one-to-one marriage. If everyone had someone as kind and genuine as my wife, we'd probably see a lot more marital stability. I am a VERY fortunate man.
Let me tell you a story. Some years ago I took a day off from work and took my wife to a very nice lunch in downtown San Francisco at the Palace Hotel. We ate in The Garden Room, which is one of the most beautiful dining spaces on the planet. Afterwards we were wandering around in one of the nearby department stores and my wife said, "I hate to shop. Let's go home."
I'm one very lucky man indeed.