A recent movement to become more accepting among the more liberal wing of the religion, which only happened by plagiarism of non-Christian moral progress, hardly wipes away either two thousand years of general practice or the existence of a conservative wing. So when a Christian claims she got her views against same-sex marriage from her religion's traditional teachings we have every reason to believe her. But if one were to claim she got views against black marriage from her religion's traditional teachings we'd have no reason to believe her -- she'd obviously be lying and just trying to game the system.
So 250ish years of "we the people" using religion to justify slavery isn't considered traditional. Ok
How the heck are you deducing anti-marriage from pro-slavery?
"MARRIAGE. -n. The state or condition of a community consisting of a master, a mistress and two slaves, making in all, two."
- Ambrose Bierce

Christian churches have been marrying slaves and preaching the virtues of marriage and the evils of non-marital sex to slaves for the same two thousand years they've been doing it to free people.