I had the same thing happen to me when I posted about how unbridled capitalism requires external costs in the form of pollution and what not. I didn't make it clear that I thought it was a terrible truth.

Once the hive decides you're being serious without checking, they turn the down vote button into an I disagree with you button.

This is actually one of the reasons I left Reddit. I hate to see it here.

It likely helps to take in the cultural moment or context around the statements or the nature of the statements you're making. It's fine to state a fact but it's also helpful to make it clear whether you are saying "it is what it is " or "I wish things were different" or "I am doing X, Y, and Z to try and help and I recommend others do so". Jokes are an exception and I think misunderstandings are fine there. But it's unreasonable to think that on the Internet, people will "check to see if you are serious".