I think that one of the things that frightens fundamentalists of any sort is the notion that there ARE choices in life. One of the attractions of a fundamentalist religion of any sort is the abdication of personal responsibility for making decisions about your life. If the rules are laid out for you, and you are told "This is the Truth and the Way you must live your life" it gives one a sense of security of not having to make moral decisions one' self.

Unfortunately, once you have decided to follow that route, it becomes extremely threatening to your world view if there are people who have made different choices. It starts that little niggling voice inside your head that says "Maybe I didn't have to do it this way...what if...". The only way to defeat the little voice is to loudly deny the validity of any other choices and to try to prevent other people from choosing differently than you have yourself. This has nothing to do with religion, per se, and everything to do with human psychology.

I think what fundamentalists find the most threatening about the dreaded "secular humanism" is that humanism requires you to make moral choices and behave ethically toward your fellow human beings because it is the right thing to do, and a way you have personally chosen in order to make the world a better place and life worth living, rather than having rules imposed from above by a judgemental Creator who will punish you if you aren't a "good" person - "good" being defined as someone who follows all the rules established by the other human beings who run the particular religious institution to which you adhere.

Those humans who set up the rules base them on their own interpretation of the writings and philosophies of other human beings from the past, and cloak their personal decisions and interpretations by saying THEIR Truth came from God. Sometimes they do it in all earnestness and religious belief, but far too often the leaders who make the rules do it with some secular purpose in mind, which has to do with acquiring power over other people in a political or psychological sense.

The poor sheep in the fold then follow their shepherd's direction, burning books or stoning women, trying "witches" or gassing Jews, because they have abdicated from the frightening responsibility of thinking for themselves.