In her non-response to the student in Iowa, Michele Bachmann simply asserted that exclusively heterosexual marriage is the "law of the land" and that is the reason it is right. The only factual (as opposed tological) problem with this is that in Iowa, same-sex marriage is the law of the land.
Bachmann doesn't seem to know this, which, at this point, is not exactly a surprise. But same-sex marriage in America is no longer an idea or a proposal. It is a fact in many states.