Since the Christian founding of this country,
You see. There's your problem.
I'm actually not going to argue with you that this was a primarily Christian nation at the outset. I'm on board with you there.
But, we are a nation that VOTES and SHOULD believe in Liberty. Since, like it or not, there is no such thing as a nation with a static never changing population, handing the govt the power to determine what behavior is "OK" only sounds good in a snapshot.
It starts to look like arse when you realize that you being in the majority isn't necessarily a permanent situation.
This is why valuing liberty is so damned important. Govt is loathe to give up power in an area once it's been acquired. IE, had we never been stupid enough to hand govt power in the marriage arena, we wouldn't even be chatting today. But, we did. See where it got us?
EXPANDING that concept to one where govt supports with is "right and non-sinful" is a pretty fricking scary concept unless you can be ONE HUNDRED PERCENT CERTAIN that you'll never be on the short end of that stick.