It's common to say that people marry for love than pragmatism in the modern age, but this article gives it an economic grounding, drawing on work from Gary Becker, and argues that another reason for the rising acceptability of same sex marriage, beyond the endorsement of one Shawn Carter.
Rather than "opposites attract" being the dominant marriage model (opposites referring to the optimal productivity of having one parent working full time and one being the full time child rearer), now both spouses can work and lead similar lives.
Viewed through an economic frame, modern partnerships are based upon “consumption complementarities” -- the joy of sharing things and experiences -- rather than the production-based gains that motivated traditional marriage. Consistent with this, co-parenting has replaced the separate roles of nurturer and disciplinarian.
We have called this new model of sharing lives “hedonic marriage.” These are marriages of equality in which the rule “opposites attract” no longer applies in the same way, because couples with more similar interests and values can derive greater benefits. So likes are now more likely to marry each other.
If hedonic marriage is the predominant model, same sex marriage isn't much different in structure.
Perhaps not the most suitable material for a wedding vow, but another source of momentum for what seems to be the inevitable acceptance of gay marriage in the near future.