Please go to your preferences page and make sure your "See Mature Topics" setting is set. Setting it to "Yes" means you see the Mature Topics forum (contains political and religious debates). Setting it to "No" means you do not see those threads.
Don't worry if you come and go. On most forums I have ever been on for anything (all except this one) I have either gotten bored of them in which case I pop back occasionally or didn't like the people in which case I left and didn't come back. I have even had one or two periods where I didn't do much on here back around May and June. Some people aren't forum people.
Don't worry about it. I have an idea, find the one thread that most interests you in a day, and post to it, saying as much or as little as you want. Once you are doing that without thinking, try moving up to two threads. Slowly build it up to a point you are comfortable with. I know a lot of people here either post loads and loads or not very often at all, and there is nothing wrong with that. Just do what is comfortable.