The core of human law and governance goes back to men like Moses,Noah and Hammurabi. How do laws like "You shall not murder " or "You shall not steal" which transcend even Jewish and by extension Christian jurisprudence sound like extremist sentiment? These laws like the ones that they came with form much of the legal system that the west added to have been tested and tried and proven as just and equitable. Marriage laws are amongst them as well.
The fact that murder and thievery should be illegal, and were codified as such in ancient scripture, has nothing to do with marriage. You can't say that something is right in some places and is therefore right as a whole.