http://en.wikipedia.org/wiki/Sex-positive_movement -- "sex-positive" is a term describing a movement meant to embrace rather than repress sexuality. The play on words of "sex-negative" refers to the opposite of sex-positive, ie., sexual repression.
OP is saying that you are sexually repressed because sex jokes make you uncomfortable. This is a good point, because dongle jokes are actually not "sexist", they are simply sexual. Men hold no exclusive territory over dick jokes, or sex jokes in general. Women can tell dick jokes too. They can also joke about vaginas (Sarah Silverman much?) and both genders can appreciate the humour EQUALLY (yay equality!). Those who don't find it funny are NOT victims of sexism-- they are victims of a shitty sense of humour.
I like sex, I just happen to prefer my work/professional environments without sexual overtones.