Why being puritanical and quick to moralize is (somewhat ironically) itself immoral.
Often people misspeak or aren't fully able to articulate what they mean. If you are quick to moralize, it is rather likely that you will shame someone over a miscommunication.
In general people are way too confident in their ethical beliefs. We are all fallible. It is likely that some of your current ethical stances are unkind or inconsistent with other things you believe.
You need a lot of information to make an informed and responsible moral critique. A couple might seem be having an unfair dynamic, but they might be in a BDSM relationship. They might have made an unusual trade. One or both of them might have unusual preferences. If you didn't see the negotiations that happened behind the scenes, you don't know if there is actually an unhealthy dynamic going on. It is not acceptable to moralize unless you know a situation with some depth.
Not everyone in your community agrees with your ethical stances. If someone feels unjustly shamed by you, y'all disagreeing and having an argument in public would create an unpleasant scene. If you have a problem with what someone did or said, it is usually much better to first privately message them with your grievance.
Shaming people is a blunt instrument, and you should probably try using other instruments first.