An interesting rant, OP ...
Short answer: It's in the New Testament. Paul writes to a church divided by sinful members ... e.g., one man is sleeping with his father's wife. Paul's response is direct: The man knows what he's doing is wrong, and if he does not change his ways, he must be ostracized from the church. I also have the impression that there is a certain dislike here, but it may not necessarily be extant.
Secondly, also in the New Testament, someone (Paul? Jesus? been a while since I've read it) teaches about correcting your brothers when they are sinning ... sadly, many focus on this lesson to point of neglecting Jesus' parable of the blind leading the blind ... So they think homosexual actions are sinful. (Essentially, the lesson is, talk to them about it, and with witnesses.)
I'm not being very coherent at all, and my post here isn't very useful without citations, and it's probably already been said ... (I post because I've seen repeatedly that some common-sense replies aren't always posted as quickly as they should be.)
Another poignant fact is that people like to feel important. If you can bring others' faults to their attention in this context, it makes you feel valuable to God, useful to each other. "Here I am, doing something! Helping this person, or trying to, anyway." It's certainly one reason I come to web forums; I wish to share any insight I may have with others asking questions ... although I usually don't have anything useful to say. And, of course, the other aspect is to feel better than others, although this is something we try not to think about and to avoid ... (as pride is sinful.) I think pride is pretty uncommon, though... I don't think there are many here malicious enough to enjoy thinking of others suffering for all eternity, and it is certainly not Christian to do so.
I guess I should learn to stop posting after 11pm; when I'm tired my articulation sucks, and I tend to ramble. Hope this post wasn't a waste of our time.