Comparisons to our attitudes toward Shabbos descecrators and Mormon polytheists are inapposite. People who don’t observe Shabbos don’t do it because they don’t believe in the Torah, or the Oral Torah. It’s an issue of revelation. Similary, Mormons don’t think there’s a rational argument for polytheism; they just believe it because they think it’s part of their revelation from God.
Homosexuality is different, and poses a dire ideological threat to traditional Jewish belief and observance. This is because they have a logical (though incorrect) argument. They think that homosexuality is benign and causes no personal or social problems (again incorrect, but they’re unwilling to look at the data objectively), and that if consenting adults want to do something and it doesn’t hurt anybody then society should not only tolerate it but treat it the same as more traditional relationships.
Even if it is wrong even on secular terms, this argument has powerful appeal to many people, including many Orthodox youth. The problems is that once they believe that, it is very hard to believe in the Torah and Yiddishkeit with complete faith.
If people opposing gay marriage are hateful, fearful bigots, then why isn’t the Torah hateful and fearful and bigoted (c”v’s!)? Without a doubt, many young Orthodox Jews who have been exposed to liberal gay marriage arguments are going off the derech because they think the Torah is wrong not to support these lifestyles. (I’ve already met once such person, a teen who apparently went off the derech due to this issue, even though it didn’t affect her personally).
Now that the Supreme Court has took their side, saying it’s a basic human right and anyone else is a hater, this view will spread throughout society, making it harder for young Jews to avoid being exposed to and convinced by it.
We need to insulate our children from modern politics and culture, including from unsupervised internet access, as much as we can. Even an hour or two on liberal websites could seed their minds with doubts and heretical beliefs. We should all consider making aliyah as well.