Why do I have these obsessive thoughts?
There's still a bit of debate on the absolute exact part of the brain which causes OCD, however, scientists have recently discovered that it probably has something to do with the immune system. What is almost universally agreed in is there's a part of your brain which controls the flight-or-fight response which is misfiring. Instead of warning you about an impending lion attack or a noise in the dark, your brain has incorrectly attached this primal level of anxiety to a particular thought or question.
Why won't they go away? What do I do?
The thoughts don't go away because you unwittingly legitimize them!

(From OCDOnline.com)
It's important to first of all note that YOU CANNOT CONTROL THE THOUGHTS YOU HAVE OR HOW MUCH ANXIETY A SPIKE CAUSES DIRECTLY. But you CAN influence it over time. The response which is only natural is to try and neutralize the fear and escape it. For example, the instinctive response would be to:
- -Google information about illnesses to confirm that you don't have them.
-Obsessively evaluate your level of arousal to determine if you're gay.
-Avoid anxiety triggers which cause you to have scary thoughts.
-Obsessively 'problem solve' to try and find an answer to an ambiguous question.
The problem is every time you engage in the instinctual response, you've alleviated your anxiety at the cost of legitimizing the fear you're experiencing. You've sent a very clear message to your brain that this particular thought is a legitimate threat, and to watch out for it in the future.
So how should you respond, then? The therapeutic response is also known as the 'extinction response'. Rather than trying to escape these thoughts, you accept them and you choose to live with the ambiguity. You allow them a place in your mind, and you allow yourself to feel the anxiety they bring. You're not giving in, and you're not going to try and get rid of the thoughts. By being accepting to your anxiety and these scary thoughts, you're telling your brain that they're not important. Over time, the thoughts and anxiety gradually drop off! The most effective way to utilize this technique is something called exposure and response prevention therapy. You are intentionally exposed to your fears based on a hierarchy, and you endure them until your anxiety level goes down. As the therapy continues, you move up the hierarchy until you finish the list you've created.
How do I know if this is OCD or a legitimate fear? How can I be certain? What I'm really losing it?
Dr. Steven Phillipson says it best in this case. Paraphrasing: "If you don't know if it's OCD, it is. Treat it as such." It's only natural with OCD to have constant doubts about your thoughts, including whether or not your thoughts are part of OCD, and it's important to note just how comprehensively deceptive and frightening OCD can be. You can feel emotions, sensations, and have thoughts which are terrifying and out of place, which is why it's only natural to try and escape them. I almost a-liken it to a dream-state; while you're in the dream, things which are completely bizarre and illogical seem very real until you wake-up. Someone who has contamination OCD is experiencing the profound sense and terror of being surrounded by dangerous germs, while a person not engaged in this obsession virtually no such fear and regards it as silly and irrational. A contamination OC knows on some level that their fear is not something rooted in reality, but they don't feel it like someone who doesn't share this obsession.
Why am I worrying about such morbid things? There must be something wrong with me!
The particular obsession in OCD has no bearing on who you are as a person, and has no clinical significance. It doesn't matter if you're worried about harming people, germs, or sexuality, the methodology if treating the disorder is exactly the same, and no clinical significance (to my knowledge) has ever been found correlating with a particular OCD obsession. It's really a waste of time trying to find the root of the obsession, and doing so really isn't therapeutically useful. Your main goal shouldn't be "how did I get here?" but rather "how do I get rid of this?"
I've personally had obsessions that run the entire gamut.

I think that if I just find the right answer, I can shut this obsession and fear off permanently!
I'd like to share a story which illustrates why this isn't the case. When I was 11, I was watching Jerry Springer on TV late at night while trying to fall asleep and I had the sudden thought of "I'm gay". This triggered a very sudden and profound sense of terror. I got up and bolted out of my room and into the bathroom where I had a major panic attack. I had no idea, but this would be the start of HOCD for me.
"Why can't I get rid of these thoughts? Why can't I stop thinking about this?"
Every time I 'solved' an anxious thought in my mind, another one would pop up 10 minutes later. This kept going for months, and I was deeply unhappy and terrified. I had no idea what OCD was, and neither did my parents. They called me a worrier (or affectionately, a worry-wart) and told me not to stop wasting my time. They bought me a book bout puberty detailing how homosexual thoughts at a young age were totally natural, and I would go and read that page of the book perhaps 20 times a day as an escape response. There seemed to be absolutely no end in site, and my parents decided to come up with a crafted lie. They told me that when I was born, they had a special chromosome test done which identifies if the child is going to grow up gay or not and mine came back negative. I was elated! I finally had my perfect answer. I could finally relax... For a day. The fear came back, which made no sense! I had my perfect answer and this fear was still going on?! How can this be?! It didn't matter, and I worried for another month or so. Eventually, my fears morphed. I started worrying about bizarre things such as if my mother was a hermaphrodite or that I was going to contract rabies or get a brain-tumor. As an adult now who's experienced with OCD and knows how it functions, it makes total sense. The reason the answer my parents gave me didn't shut the OCD off was that it's impossible to give OCD a perfect answer, because OCD is by its very nature illogical. It will bend and contort in ways which don't play by the rules to perpetuate the obsession, and using stone-cold logic is frankly a waste of time for any long-term relief. The only way to win is to not play its game.
Would talk-therapy help me through my OCD?
The short answer is no, because talk-therapy is an attempt to reason with something which is inherently unreasonable. Historically it's been pretty ineffective at treating OCD, because the person doesn't need convincing that the fear is irrational. Though plagued with doubt, OCs know that their fear is irrational or at least the time spent obsessing is irrational and they'd like to be free of it. Exposure and response prevention therapy from a skilled experienced behavioural therapist is the best way to treat OCD.
What are some good resources for OCD?
These are my favourite two, and they're both from the same person. To give you an idea of how prolific this guy is in the world of OCD, he invented the word 'spike' and has really been the one to pioneer the major treatment for Pure-O OCD.
2 HOUR VIDEO SERIES ABOUT PURE-O OCD (and OCD in general) FROM DR. STEVEN PHILLIPSON:
http://www.youtube.com/watch?v=uhVp4g742kM
LIST OF OCD ARTICLES FROM DR. STEVEN PHILLIPSON:
http://www.ocdonline.com/articlesphillipson.php
Any Advice?
-Don't suffer alone. Tell someone close to you about your illness, and if you're comfortable, tell them the specific obsessions you have. Having someone know exactly what you're going through has helped me immensely in my experience. Try to educate those close to you about OCD and show them videos and articles.
-YOU CAN ENDURE THE ANXIETY, AND IT WILL GO AWAY ON ITS OWN WITHOUT THE ESCAPE RESPONSE. The moment you realize that you don't have to do what OCD tells you, you're going to gain a new sense of freedom. You can't control your anxiety or your thoughts, but you can damn-well control your own actions. It's taken enough from you already, and you're not going to play its game anymore.