when the line between science and advocacy is blurred

From Same-sex parenting and children’s outcomes: A closer examination of
the American psychological association’s brief on lesbian and gay parenting:

However, a closer examination leads to the conclusion that strong, generalized assertions, including those made by the APA Brief, were not empirically warranted.99 As noted by Shiller (2007) in American Psychologist, ‘‘the line between science and advocacy appears blurred’’ (p. 712).

 How does objectivity go out the window so easily?  Within the same article there is a study that shows one notable area in which being raised by a same-sex couple:  Social Studies Achievement.

A lot of such people are going far in Social Studies.

This reminds me of the article in which Jonathan Haidt grapples with the near homogeneity of the social sciences with regard to politics.  Tell me ... If 90% of the social studies departments were politically conservative, would most liberals accept their findings as scientific?   Would the lack of diversity be grounds for skepticism for many?  Would they call it science or advocacy?