Assuming that Sesame Street and Nova are the LEAST liberal programs on PBS don't you find it interesting that multiculturalism is so predominant on Sesame Street and that it just happens to be an issue that is far more important to the left rather than the right? Multiculturalism is a good thing and age appropriate too. What about two daddies and a boy? That is controversial and not age appropriate.
Doesn't Nova also present far more programing in support of the kind of science that is favored by the left rather than by the right?
I am not saying that other types of programming ever exist in these shows. I am not saying that the programming they have is not good. But a lot can be done by presenting weighted or one sided programming.
If "multiculturalism" means accepting that people who are different from ourselves should be accepted as equals, is that really a "liberal" idea? Sometimes, the meaning of words like "multicultural" and "liberal" get blurred, but if the opposite, non acceptance of people who are different, is a "conservative" idea, what does that mean about conservatism?
As for science, there is only one kind. Science is not "liberal" nor is it "conservative", not by any reasonable definition of those terms.
So, we are left with a depiction of homosexuality as normal, the one controversial issue you've brought up.