Over the millenia, marriage has always been for the purpose of recording immediate and extended family relationships according to blood lines. In the past that was to establish ownership of property, heritage and some political considerations.
In more modern times such criteria still has some importance but marriage is specifically and necessarily regulated in all 50 states to protect children with requirements for legal age to marry, prohibiting too close family relationships to marry, establishing what persons the adults are responsible to care for, right of inheritance, certain medical considerations, etc.
The Constitution gives the federal government no jurisdiction whatsoever to regulate marriage. But as traditional marriage produces tremendous benefits for both adults and children and communities as a whole, the federal government is justified in establishing a tax code, etc. that encourages and supports it.
In my opinion there are efforts to weaken and perhaps destroy traditional marriage as the norm in America to further weaken America. And yes, that is a conspiracy theory that I suspect will eventually be shown to be fact, not theory.