They can’t. But I think there are some things which we can learn from nature/evolution on these matters. I am not sure everyone in this conversation will enjoy hearing what nature/evolution says though.
I am not the first to say it, but the nuclear family is the only human institution that can create new life, and keep that life in daily communion with its creators. The nuclear family is a near-universal bedrock in every culture in the world. A non-anthropologist would be hard-pressed to name even one nation where any other arrangement for conceiving, nurturing, and raising children into adults was considered the optimal social arrangement.
Now a decent anthropologist could name some tiny cultures where other arrangements are the norm, such as the “mother’s-brothers” type where there is no father figure and the brothers of the mothers are the role models for young men. They might cite an example or two of truly communal living. So other arrangements besides the nuclear family have been tried, and the results are in. The thing is, these examples are so rare, and the cultures they produce so backward, that the exceptions serve to prove a rule: the traditional nuclear family is the superior social structure for building a healthy society.
The nuclear family forms spontaneously in all successful cultures regardless of the race or religion of the society. Other family forms which usher in the next generation (and thus continue the society) are so uncompetitive with the nuclear family as to be nearly extinct. Note that a nuclear family can take two forms: monogamous or polygamous. Of the two, cultures espousing the former have had greater success in producing advanced, healthy societies than the latter, and even in those cultures where polygamy is practiced by the upper class, monogamy is more common even if only as a result of biological necessity (male and female babies are born in about equal numbers).
Marriage is what binds men and women together to produce a nuclear family. Given the essential historical role of the nuclear family in producing a healthy and well-adjusted next generation, it is crystal clear that the state needs marriage more than marriage needs the state.
In our nation there is a culture clash of two major world-views. One is secular, naturalistic, results-oriented. The other is Christian, faith-based, theistic, and more willing to look beyond immediate results in order to make choices in agreement with their moral code.
On most issues, these two groups clash. On this issue, they shouldn’t. On this issue, it does not matter whether you believe Man is the result of a billion years of evolution or you believe that Man is the creation of God Almighty. The bottom line is that same, one social arrangement is clearly the “right” one for society to uphold with the exalted recognition of “marriage” - the nuclear family.
If you believe that Man is the creation of God and that marriage is ordained of God to be between one man and one women from the Garden on then you agree that the traditional family is the best option for a healthy society. If, on the other hand, you believe we are simply the product of evolution then the evidence demands that you believe that humans evolved to be socialized from a foundation of a nuclear family.
Whether we are the creation of God or the product of evolution the bottom line here is the same: nuclear families do the best job of successfully producing a succeeding generation to continue the kind.
Marriage is an exalted status given by society to a social status that both God’s law and scientific study has shown are best at sustaining a culture and its people.
Notice that marriage is not about how you “feel” about your partner. It is not about what you and your partner consider each other. Marriage is about society’s sanction, approval, and recognition.
Evolution and scripture are telling us the same thing on reproduction and child rearing. Adam and Eve were the example that mankind was to follow, and even they were pointing to the archetype example of Christ and the Church. We could view it as nature/evolution confirming scripture or scripture affirming what we learn from nature/evolution.