From my own knowledge, this rests on a single verse saying "a man shall not lie with another man", or something along those lines. That's the entirety of homosexuality in its own right being "un-Christian", with another in the definition of marriage. By that bar, we can tell you that you're being "un-Christian" by demanding the state enforce your morals. "Render unto Caesar what is Caesar's" and such, a greater degree of separation than you're assuming the Founders had.
It's actually a series of verses throughout the Bible that say that homosexuality is immoral. Particularly in Levitical law and a couple places in the New Testament.
Where along the line is it deemed necessary this be explicitly religiously Biblical? A shared ethical framework performs the job perfectly well, even if you have a third of the population be Muslim, half Christian, and the remainder evenly split between Hindu, Jewish, and Shinto. The aspect that matters is a shared morality, not the justification of that morality. "Because the Founders said it" and "Because Jesus said it" are no different in this function, even if the former came from the latter.
You're showing that you don't know what you're talking about here.
Muslim morality is not the same as Christian, Jewish, Shinto, or Hindu Morality. Shinto morality likewise is not the same as the others, and neither is Hindu. Jewish and Christian morality have a lot of overlap, because the one is an outgrowth of the other, but even then there are significant clashes.
Should we be living in a caste-based society, where what family you are born into determines what your social standing and profession will be? Where if you are low-caste, you deserve it because of your sins in a past life? Where if you are high-caste, you deserve that privilege for your virtue in past lives? Where burning the widow of a dead man alive on his funeral pyre is acceptable?
Should we be living in a society where honor killings are acceptable? Where it is virtuous to kill those who refuse to convert to Islam?
Should we be living in a society that looks like Japan pre-Hiroshima and Nagasaki?
Should we be living in a society with an atheism derived morality, like those famous exemplars of virtue, Pol Pot, Mao Zedong, Lenin, Stalin, Hitler, Castro, Chavez?
Or should we live in a society where we are not only supposed to love our family, but also our neighors, and even our enemies?
Because let me tell you, of the religions listed, only
one teaches that, and it's Christianity.
I get it. Christian values yadda yadda yadda. Do you know what the country was founded on besides chrtisian values? Freedom of Religion, freedom of speech.
Freedom of speech and freedom of Religion
are Christian values. They aren't
exclusively Christian, but Buddhism is the only other world religion that professes anything like such, and it has some very substantial other issues.