I believe that the term "Conservative Christians" as I use the term accurately describes a group of individuals that, despite the natural variability within any large group of people, generally possess the following characteristics:
1. Evangelical Protestant
2. White
3. Socially conservative, and thus (again generally) opposed to progressive social reforms related to, for example, civil rights, women's rights and gay rights.
4. Votes republican, and indeed forms a core of the Republican voting base
Where am I wrong, and how is this characterization prejudiced?
I generally don't consider Mormons when I refer to conservative Christians. Although Mormons do share many beliefs on social/economic issues with conservative Christians, outside of Utah and perhaps Idaho, Mormons are not a powerful voting bloc and thus their potential for mischief and putting up roadblocks to bringing socially and economically marginalized groups in the the mainstream of US society, and denying civil rights and liberties to marginalized groups, is much, much more limited.
Having spent much time among both Evangelicals and Mormons, I'd choose the company of Mormons in a nano second. While they have similarly whacky beliefs, Mormons tend to compartmentalize much better and also tend to respect boundaries much better.
As for Mormon opposition to slavery, it does not appear quite as cut and dried as you make it out to be. Take the following quote by Brigham Young found here:
https://mit.irr.org/brigham-young-we-must-believe-in-slavery-23-january-1852. I admit, however, to not being particularly well-informed on this particular topic.
"I have this section in my hand, headed “An Act in Relation to African Slavery.” I have read it over and made a few alterations. I will remark with regard to slavery, inasmuch as we believe in the Bible, inasmuch as we believe in the ordinances of God, in the Priesthood and order and decrees of God, we must believe in slavery. This colored race have been subjected to severe curses, which they have in their families and their classes and in their various capacities brought upon themselves. And until the curse is removed by Him who placed it upon them, they must suffer under its consequences; I am not authorized to remove it. I am a firm believer in slavery."
If I have mischaracterized your position on anything, please forgive me. In my defense, your posts tend to be long and rambling, so it can be hard at times to pinpoint what you believe.
Finally, progressives behind the times on social issues relative to Conservative Christians? That, my friend, is laughably wrong.