Basically, schools teach kids concepts about family and marriage that include gay and lesbian people and people who disagree are seen as bigots. It's not about protecting gay and lesbian couples (or, when they exist, the children of gay and lesbian parents). The concern is that marriage equality requires others outside of the gay marriages to consider that marriage is more than men and women coming together.
In other words, the harm isn't anything that married gays or lesbians have actually wrought on society. It isn't even that married gay or lesbian people have actually brought financial or physical harm upon society. It is all about the feelings of those that disagree with marriage equality. Gay marriage harms then solely because it exists.