>>1
Sick? No.
Should never strive to get to experience bukkake IRL thanks to all of the STDs in the world? Yes.

Fantasies such as that work as a way of acting out emotions and problems inside of yourself. It's not sick, but you do need to do something about your insecurity (yes, I'm being Captain Obvious).
Your existence does not have to be justified, you have every bit as much of a right to exist as anyone else.

Hmm, try getting some help with how to deal with insecurity and the feeling of lack of self-worth (why else would you feel you have to justify your existence?) here for instance:
http://stevepavlina.com/forums/ The forums have a lot of clever, helpful people who often know exactly what they're talking about. Reading random threads probably will prove to be quite beneficial as well.
You probably should look into personal development. Lifehack.org tends to have practical articles. Steve Pavlina's PD blog (one PD blog of many on the vast intarwebs) is quite useful as well, though has gotten a tad hippie-ish lately. That's not a problem though: the archives are filled with useful, practical stuff.
random possibly useful article: "Five Principles for Happiness in 2007"
http://finance.yahoo.com/columnist/article/millionaire/18930
if you read articles anywhere that has comments, don't forget to read them.
You also should consider going to either therapy or similar. It may be useful to have someone professional help you. Do remember that there are many different styles amongst therapists, and that they're essentially human, even if extremely qualified such. So if one doesn't fit you, that doesn't mean that therapy in itself doesn't fit you. I think cognitive therapy might be good for you.