Di,
1) Prohibiting homosexual practices and the separation of church and state are two different topics. The First Amendment is what separation of church and state is based upon, and the First Amendment does not prevent the government from prohibiting homosexual practices.
Can you point to any court cases which ruled that an anti-sodomy law was unconstitutional based on the First Amendment's free exercise of establishment clauses? As Gregory pointed out, what the courts have relied on when dealing with these kind of laws is the alleged right to privacy, not the First Amendment.
2) If the courts and society remember their roots, if they remember that Roger Williams (when he got religious and civil liberty started in this country) taught that the government can only enforce the 2nd table of the 10 Commandments as it pertains to outward actions, but is forbidden by God to enforce the 1st table, there won't be any problem whatsoever when it comes to the Sabbath.
The real problem is that too many today don't remember their roots, and thus don't remember why they can't pass and enforce Sunday laws.
Remember that Ellen White multiple times called the U.S. government a "Protestant government." Some Adventists who take the concept of separation of church and state to extremes would be so audacious as to say that Ellen White was wrong when she wrote that. But the fact of the matter is that it is because the U.S. government is a Protestant government that we have liberty of conscience and separation of church and state.
If liberty of conscience and separation of church and state must solely be based on human reasoning and never upon Scripture, then it can be tossed out of the window at some point by society. But if it is also based on Scripture, it can't be tossed out of the window by a Protestant government without constituting apostasy and inviting divine wrath.
3) Roger Williams was neither a Republican nor a Tea party member. Neither was the apostle Paul, Ezekiel, and Matthew, who wrote the Bible texts that A. T. Jones used in his 1889 booklet National Sunday Law when explaining why the government can only enforce the 2nd table, never the 1st table of the 10 Commandments.
4) "Are you saying that you want them to intefere when it's homosexual relations, but it's none of their business when it is the relations of heterosexuals in their bedrooms." The government already interferes. The government prohibits polygamy and incest, even when the adults involved are all consenting. Is that wrong? No, not according to Paul, Ezekiel, and Matthew.
5) The real slippery slope is abandoning and rejecting the historical and biblical basis for freedom of conscience and separation of church and state.