A widely-noted exchange in this week's Supreme Court marriage equality cases involved Justice Antonin Scalia's concern that gay unions would be damaging to the couples' children. He didn't mention any specific harms.
The experience of the Netherlands -- the first country to legalize both same-sex marriage and adoption by same sex couples, in 2001 -- suggests Scalia was getting ahead of himself. Statistics released by the Dutch statistical service five years after legalization, in 2005, found that gay couples were still four times less likely to marry than straight couples, and that continued difficulty adopting children was a possible factor. A demographer with the Dutch office at the time told Expatica:
Many [same sex couples] marry out of love, of course. But, just as with straight couples, the desire to have children and the resulting legal responsibilities often weigh even more for gay and lesbian couples.
This could explain why gay couples marry less often than straight ones. Gay couples, especially gay men, still face considerable difficulties when they want to adopt children.
Five years after legalization, only 20 percent of gay couples had married in Holland, compared to 80 percent of straight couples, the Dutch statistics showed.