Most things about sex are a matter of preference and opinion

This isn't true. Sex is biology and psychology. It is a science. That's why we have sexology.

And the only reason they are controversial in the first place, is because those personal opinions have no need to be proven or disproven scientifically

Again false. These "opinions" are based on reasoning, and that reasoning can be true or false. If somebody said they don't like promiscuity for no reason at all they just randomly decided on it then that would pure opinion, but almost pretty much nobody says that. They always have reasons and reasoning can be proven true or false.

Now as the for actual attitude of this subreddit I don't think you are accurately portraying it at all. People are free to say what they personally do or do not like, without making blanket judgements about the morality of it.