Christians have claimed that Christmas, marriage, morality, and more are theirs to define and control. What unites these issues is an effort by conservative Christians to claim ownership over cultural or political institutions which should be open equally to all citizens. They don't want to be mere contributors to a larger whole, they want to be owners with a right to exclude others. This is basically and expression of tribalism and attempt to exercise territoriality, not unlike what dogs do.
Property represents power, so the distribution of property in society determines the distribution of power in that society. When property is held by only a few, then power is also exercised by a few and this is antidemocratic no matter what the formal structure of the political system is. When the ownership of property is widespread, power is also spread throughout society. This is not just true about physical property like real estate, but also political and cultural institutions "property" is most fundamentally about being having the authority to control a thing and exclude others from using that thing.
When more people are admitted as equals to institutions like marriage (or, to put it another way, when more people are allowed to claim "marriage" as their own), then cultural and political power is distributed more broadly through society. When institutions like marriage are limited to a privileged group, then that cultural and political power is restricted to them and concentrated in their hands as well. That's the point of concentrating property and wealth into fewer hands: limit the power to as few people as possible in order create a more strictly defined social hierarchy where a few can make decisions for many.
It is not legitimate for Christians to try to stake out something like Christmas for themselves alone, it is not legitimate for conservative religious believers to stake out marriage as something which they have the sole authority to define, and it's not legitimate for religious theists to stake out a political party for their own personal use. Believers who try to appropriate cultural and political institutions for themselves are acting like a puppy marking the corners of its new yard: they are engaging in territoriality by excluding "undesirables" and even defining themselves by the fact that those "undesirables" are not included.
In the end, though, all they end up really doing is peeing all over everything.