That is simply a flat-out lie.
I'm sorry for your and your beliefs but it is not. You maybe unaware of, or unwilling to accept, practices sanctioned by the Mormon church but that doesn't mean they didn't happen.
The vast majority of the specific points you mention are not, and have never been, considered acceptable within the Mormon church. You appear to be confusing the genuine Mormon church with a few of the more bizarre offshoots thereof.
Really? So Joseph Smith's acceptance of marriage between siblings (e.g. John and Maria Bernhisel) didn't happen? Fanny Stenhouse didn't say:
It would be quite impossible, with any regard to propriety, to relate all the horrible results of this disgraceful system.... Marriages have been contracted between the nearest of relatives; and old men tottering on the brink of the grave have been united to little girls scarcely in their teens; while unnatural alliances of every description, which in any other community would be regarded with disgust and abhorrence, are here entered into in the name of God
back in 1875?
What about Smith threatening young women with "eternal damnation" if they refused to marry him? Or Brigham Young threatening to ruin Ann Eliza Young's brother if she didn't marry him? (Wife no 19).
What about George Smith's study of marriage within the early LDS movement, indicating 1.3% of brides were 13, 8.5% of brides were 14 and only 42% were 17 and over? Or Todd Compton's documentation of Joseph Smith's multiple marriages to girls of 13 or 14? Or Stanly Hirshon? Who documented cases of girls aged 10 and 11 being married to men in their sixties?
Given the LDS's propensity for editing its own history (and scriptures) when it feels this necessary you may have missed these parts.
Even today the Mormon church considers women to be subject to their husbands, though it's downplaying the older doctrines somewhat but
"[a] woman's primary place is in the home, where she is to rear children and abide by the righteous counsel of her husband"
Then there's the support the 'mainstream' LDS church gives to its own fundies, who are open about continuing such church practices such as polygamy, racism, incest et cetera.
In any event, discussion of the specific claims is not relevant to this thread. This thread is about one specific evil. Claiming that other evils also take place is not, in any way, an argument in favor of a different evil.
Strawman. I'm not claiming some equivalence of evils, I'm stating that the church
whose moral authority you cite for your opinions on same-sex marriage is guilty of practices (historically and currently) that even you admit are equally wrong. Yet that church continues to evade, deny and whitewash its past while opposing marriage between consenting adults, just because they are of the same gender.
IThat some people in the commit murder is not an argument in favor of rape. That some people commit rape is not an argument in favor of theft. That some people commit theft is not an argument in favor of murder.
Another strawman.
One last question: if (when) the LDS comes out and says that same-sex marriages are acceptable will this change your opinion? It's not like this would be a greater doctrinal change than some they've made.