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.
Like even if a lot of people were searching it, you might not want to be inadvertently suggesting morally questionable queries to people who don't want to see them.
As such, it seems like they err on the side of caution with it. Like there are no suggestions for the word "fuck" despite the fact that it is a wildly popular query.
Their algorithm probably just ends up tagging "is my son gay" as a query that people can enter on their own if they want to search it.