Data from The Netherlands' national statistics agency showed 15,000 gay couples have married since 2001 [when SSM was legalized]. That means just 20 percent of gay Dutch couples are married, compared to 80 percent of heterosexual couples, the agency says.

Bergkamp sees three main reasons for the lack of nuptial enthusiasm among gay couples: less pressure from family and friends, fewer gay couples marrying to have children than their straight counterparts, and a more individualist, less family-orientated mindset among many homosexuals.

This article doesn't ask: how many gay people are coupled?  What really matters for the "take-up" rate, is what proportion of gay people (vs. those attracted to the opposite sex) marry.

This brief needs to be updated, but its core findings appear to hold.