I think I understand what mjgates means about not singling out any group. In my opinion, doing so is a form of prejudice. When you single out any one group to teach tolerance about, you are implicitely saying, one, they are weak, inferior, handicapped, or so on, and two, any group you don't single out isn't worth your attention.
Teach tolerance, of course, but do it as the subject comes up. If you don't note that there is anything odd about Johnny having two daddies, chances are junior won't either. If he does make comment, then you redirect depending on his age, maturity, and the situation. I think this is what mjgates means.
I think I also understand what mjgates means about not "teaching homosexuality." I don't think you need to bring up any sexuality to young children. But I don't think this is being done, or I know it is not being done in my child's public school and I would hope it is not being done elsewhere.
I have no problem with there being casual references in children's books to Johnny's two daddies in the same way that there might be casual references to Jill's mom and dad, but I don't think it should be taught explicitely, for the same reasons as above - it doesn't actually teach tolerance except in the traditional sense of the word, not the sense that we mean it when we use it. I also think it just confuses matters for young children.