I find that my fantasies seem to get more and more...visceral the more turned on I am. I can begin jerking off over relatively vanilla things and enjoy prolonging the experience. By the end of it, I am pretty down for licking sweat from a beautiful teen ass, being choked until I pass out, getting head from a girl on the toilet (not doing anything though), wiping semen all over my body, having my face sat on by a voluptuous curvy woman after physical activity while she helplessly squirts milk and lady cum all over me. As soon as I cum, I come down and feel quite different. :D
Its like the few times after badly timed anal when you've busted and realise you have a slightly dirty cock, when minute ago the thought of pounding that ass was totally compelling. It stops being a sexual organ and becomes a waste disposal unit pretty rapidly.
I recall reading about a study that found that our 'disgust' response declines with sexual arousal. You will suddenly find things to be appealing that were previously fucked up. Sometimes I think I am a really sick fucking person :D
The only real fantasy I cannot understand is scat or watersports. I cannot imagine a situation in which that could be anything but off putting and private.