UMASS economist M. V. Badgett has released a “discussion paper” examining the evidence from Scandinavia and the Netherlands – available in both html and pdf formats. You can also read a short summary of the paper’s conclusions here.
In essence, the evidence is consistant with the common-sense idea that same-sex marriage has no impact on straight marriage. The vast majority of heterosexuals deciding whether or not to marry, aren’t basing their decision on if same-sexers have legal recognition of their relationships.
The paper also makes this point, about how heterosexual marraiges are even less likely to be affected in the US:
The lack of support alternatives plus the tangible benefits of marriage all lead to one conclusion: if and when same-sex couples are allowed to marry, heterosexual couples will continue to marry in the United States.
Incidently, folks interested in the statistics and social-science side of the SSM debate should check out Professor Badgett’s site, which is full of interesting papers.