The indoor cats debate is the biggest and worst example of this I've seen recently.
People feel like they have to defend themselves when they are called an animal abuser that doesn't care about their pets. Most people love their pets a lot, and most people think of "animal abuser" as an unspeakably evil category of people.
I get defensive about the indoor cats thing, because I had outdoor cats as a kid and bad things happened to many of them, and I used to feel incredible guilt about that even though I couldn't have done anything because I was a kid.
The main things that people are actually getting stuck on with the concept of indoor cats are:
they grew up with outdoor cats, everyone around them growing up considered keeping cats outside normal and harmless, and it's just weird to have a Literal Stranger expect you to accept that literally every person that ever loved you or who was kind to you growing up is an Animal Abuser
they think that cats, as a species, literally need to roam around outside or they will not be having their needs met.
These people are not at all wrong to worry about how to provide enrichment to an indoor cat!!! Cat furniture and puzzle toys and ipad apps with fish swimming around for your cat to paw at are not known to everybody. There's also a persistent myth that cats cannot be trained and therefore training one to walk on a leash or play fetch is absurd.
(It also at least deserves mentioning that there are public outdoor spaces, activities and sports events meant for dogs.)
It has literally never occurred to many people that cats are an invasive species. They don't know where cats are native to. They don't know that there were no cats in their area before humans brought them there. It seems strange, I know, but you are ignorant about something that seems obvious to someone else, so please stay humble.
When you describe cats as "cold-blooded serial killers of native wildlife" or things like that, it really does sound like you are moralizing an animal being a predator. Cat lovers grew up suffocated in cutesy animal books, shows, and cartoons that demonized cats and other carnivores for being carnivores. Assigning morals to animals goes all the way back to Aesop's fables and Pliny the Elder. You have to make sure it's clear you're not doing that. 
I write this because I wrote something about the harms of outdoor cats in a reply one time and went back and read the tags on that post later, and the sheer number of people who had written that my post specifically had changed their mind because it was the first post they had seen on the subject that wasn't needlessly hateful and aggressive blew my mind.
Educating effectively requires you to think about the effects of your words. You can't just say things you Know are right and consider your job done.
