What seems incongruous with this to me is that they also filter many (non-explicit) gay-related searches.
For example, typing "is my son" yields:
is my son autistic? is my son smoking pot? is my son ready for kindergarten? is my son gifted?
Doing a google trends search for these four phrases along with "is my son gay?" shows that the gay query is dramatically more popular: http://bit.ly/aPNHxO
It seems like in some of those cases, knowing that others are searching the same thing could provide some comfort. I'm not sure why Google filters these queries.