don’t worry, i don’t think your question is rude or anything. it’s actually a common question and there is no harm done in respectfully asking for information.
in reading your question, i found it odd that you find a desire for sex difficult to understand because arousal would be involved. is this because you believe that asexuals don’t experience sexual arousal…? and you believe there to be a link between arousal and sexual desire, i assume.
i can only really answer your question based on my personal beliefs, which some will disagree with, but to me sexual arousal and sexual desire on their own are independent of one’s sexual orientation. that is, arousal and desire can play a part in one’s sexual orientation, but both can also exist independently of orientation and have no really baring on orientation.
sexual arousal is a physiological response to stimulation, be it physical or mental. sexual arousal can be experience by anyone regardless of their sexuality, although not everyone experiences it the same way if at all.
sexual desire is a desire like any other. that is, it is a psychological want for something (in this case, sex), regardless of whether the person actually acts on that want/desire or not. contrast this with sex drive/libido, which is physiological like sexual arousal; it is a physical, uncontrollable urge that some people have and others lack, whereas sexual desire is a matter of the mind.
people in the asexual community often have very strong opinions on what sexual desire is or isn’t and it’s relationship with sexual orientation and i know that some will disagree with me on this, which is fine of course. but to me, a desire (or lack thereof) for something in and of itself does not affect one’s sexual orientation. a self-identified heterosexual, for example, can be sexually attracted to someone of a different sex but still have no desire to act upon that attraction, be it because of sex repulsion or any other reason. that person’s lack of desire for sex does not invalidate their heterosexual identity, imho.
the same applies to asexuals, i believe. an asexual may not feel sexual attraction towards anyone, but may still desire sex, be it because they enjoy the sensations of it, because they want to have a child, because it is a means of being intimate with their sexual partner, etc etc. there are valid reasons for having sex that do not involve sexual attraction as there are valid reasons for not having sex, regardless of sexual attraction. arousal has nothing to do with anything, in my humble opinion…
sorry for the long-winded and probably round-about way of answering your question. i hope it helps rather than confuses! :(