Category permissions are post, reply, read.
There’s not a way to let people see titles but not the posts.
You can configure a category so that people can post, but not reply, but you can’t configure it such that they can post, but reply only after posting (unless, perhaps you created a badge that was awarded when people posted and then that badge was used to assign people to a group).
I think that what you want is to allow people to read without registration, but require them to get an account to be able to post. Sure you’ll be “giving away” your valuable posts for free, but many people won’t want to register until they know that there is enough content to make it worth their time.