The national discussion in the US about same-sex marriage is heating up. Last week, North Carolina voted to prohibit the practice, and, for the first time, President Barack Obama came out in favour of it.

The debate also mirrors a deeper generational shift in how the US views and experiences marriage. For their grandparents’ generation, marriage was about separate roles, separate spheres and specialisation.

University of Chicago economist Gary Becker won the Nobel Prize partly for describing the family as an economic institution. In Becker’s view, the joining of husband and wife yields a more productive “firm”, because it allows one spouse to specialise in earning income from working in the market, while the other specialises in the domestic sphere. The division of labour allows for greater productivity, as it does in the workplace. The different skills required for these separate roles provide an economic rationale for our grandmothers’ advice that “opposites attract”.

Naturally, couples who have bought into the traditional notion of marriage find the concept of same-sex marriage foreign. Same-sex relationships are less likely to involve traditional roles and separate spheres, as evidenced by the fact the partners are more likely to both work outside the home.

But heterosexual couples in more recent generations are also less likely to aspire to separate-sphere marriages.

Economists describe a “second Industrial Revolution” in which washing machines, dishwashers and microwave ovens have reduced the value to the family “firm” of employing a domestic specialist. Cheap clothes can be imported from China. Healthy meals can be bought from Trader Joe’s freezer.

Legal and social changes have broken down many of the barriers keeping women out of the labour market. Explicit discrimination has declined. Women have gained more control over their fertility.

These developments have increased the opportunity cost of having a spouse stay home, as that spouse now has greater value in the marketplace. As a result, our grandparents’ marriages, in which husband and wife had separate roles and spheres, are no longer so popular. Two-earner couples have become the norm, and families spend less time on housework.

One might have expected marriage to disappear as its traditional benefits faded. Instead, it has evolved. Modern marriage offers different benefits. Today, we search for a soul mate rather than a good homemaker or provider. We are more likely to regard marriage as a forum for shared experiences and passions.

Viewed through an economic frame, modern partnerships are based on “consumption complementarities” rather than production-based gains that motivated traditional marriage. Consistent with this, co-parenting has replaced the separate roles of nurturer and disciplinarian.

We call this new model of sharing lives “hedonic marriage”. In these marriages of equality, the rule “opposites attract” no longer applies in the same way, because couples with more similar interests and values can derive greater benefits.

The changing nature of heterosexual marriage has made the rite more attractive to same-sex couples. In turn, the gay and lesbian community has chosen to spend political capital advocating for greater access to marriage.

For heterosexuals who have embraced the modern notion of marriage, the idea of same-sex marriage seems natural. Not allowing these couples to marry is as arbitrary as not allowing those of different races, ethnicities or religions to marry.

Many of the opponents of same-sex marriage are also opponents of the ongoing shift to marriages of equality. Theirs is a futile battle. The reach of markets will keep expanding, allowing individuals and families to reap greater returns by selling their specialised skills and services outside the home. Technological change will further undermine the benefits of specialisation within the family. Improvements in women’s education will continue to raise the opportunity cost of staying at home.

The implication is ultimately, traditional marriages are doomed. The best way to let marriage thrive in the 21st century is to embrace the new model of equality and to welcome all couples, regardless of sexual orientation.

Betsey Stevenson and Justin Wolfers, professors at the University of Pennsylvania’s Wharton School, are Bloomberg View columnists. The opinions expressed are their own.