There was a time when homosexuality was considered morally wrong by the majority of people in this country. Polygamy was also considered wrong, as was incest, marrying children, bestiality, and probably some other things that I'm forgetting.

So my question is this: Given that the majority of people here seem to be so liberal-minded that they consider (consensual, non-coercive) polygamy to be fine, is there anything that anyone will still look at and say "That's actually morally wrong"?

You've answered your own question: "consensual, non-coercive"

First, don't put words in my mouth. That's not my answer.

Second, that's it? Nothing more? Consensual, non-coercive between, say, a 70-year-old and a 9-year-old is fine?

A 9 year old can't give consent, by law (at least in 1st world countries).