It is known if the thumbnail is NSFW it is going to get flagged. YOu also shouldn't expect to be able to force that upon people against their wishes. You are correct there is no way to do it now, but I've seen those enforcing things warn of this. So your opinion isn't going to change it. I do not think people should be flagging NSFW posts that are clearly marked and the initial thumbnail is not offensive. Allow people to opt in. Also porn does NOT bother me, but I respect the rights of other people not to see it.
EDIT: I saw a 14 year old doing an #introduceyourself a couple of weeks ago, and there are also religious people, and even perhaps others. It also appears some of the content I looked at is copyrighted. That could open a can of worms before we are ready to fight off the big fish that come looking.
First I've ever heard of it. If the post is tagged NSFW it should be fine. You don't get to choose if there is a thumbnail or not or pick which pictures are used. I've had it use the 3rd pic in a post before. I believe the devs do have plans to give you the option to enable/disable nsfw content. It's beta so for now you just have to live with it. That's what an uncensored platform is.