In the San Francisco Examiner:
The census data — which for the first time allowed gay couples to identify themselves as a family — revealed that San Francisco has 7,530 gay male couples living together, but of those, just 332 — or 4.4 percent — have children. Of The City’s 2,754 lesbian couples, 525 have children — about 19 percent.
... research showed that about 80 percent of the children raised in same-sex households come from prior heterosexual relationships, while less than 20 percent come from adoption, fostering or surrogacy.
A typical situation might be a gay man who had a child with a woman early in life, but now is raising that child with a male partner.