The United States, as most countries is secular. There is good reason for nations to be secular, which is that they are geopolitical entities which in general offer religious freedom to their citizens. States that make laws based on religion oppress citizens who do not follow that religion, so large nations like the US with a very varied permanent population should not arbitrarily make laws based on the religion of the lawmakers. Regardless of whether or not you agree with separation of church and state, the fact remains that the US is secular based on its own constitution.
State recognised marriage is a legal matter concerning what that country’s government considers as a legal marriage. Marriage in the context of legal matters is not affected by religious views, it is simply state recognition of a couple’s status and gives them the legal rights associated with that status for things such as the right to have a child together and maternity leave, inheritance, taxes, etc. The religious definition of marriage has nothing to do with this issue. Religion has nothing to do with this; it is not affected by religion and it has no effect on religion. There have always been people who other religions view as sinners, so when homosexual couples get married and other people view it as a sin, it is not a new thing. The only difference now is that the religious people can see this sin happening around them and feel the need to impose their beliefs on others.
If you don’t want to accept their marriage within your religion, that is a different matter, but same sex marriage is a legal matter that has no concern for religion. Religious people who want to change the law because they see an opportunity to force others to follow their beliefs might think they are doing it for the greater good, but religions often promote letting others be and reserving judgment for their god. When people choose to ignore these beliefs and instead try to change a nation’s laws, it is analogous to although not as extreme as militant groups such as ISIS.
Whether a government considers two people as married has nothing to do with whether people of a religion consider them as married. Let couples have their government based legal rights, no one is forcing priests to conduct gay marriages.
State marriage is a legal matter, religious marriage is a religious matter.