Writing is never a pane of glass but I think purley erotic writing has less curtains than most.
Sexual responses start out innate and become Pavlovian from there, it started to reach operant and abstract levels for many people but that ballast tends to get lost in the dark once the clothes come off. So when talking of the purely erotic goal is isn't much different different from showing a hungry man a photograph of a pie--except that is it a physical need that can be partially met without the "food".
The brain as a sex organ, well. You can add a 'brain' level to erotica to enrich it, but most people devote more of their brain to their inhibitions than to their fantasies. As far as sex organs go, genitals are much more straight forward, really. If a certain man's main sexual organ is his brain I can only conclude he is be doing more thinking that ****ing
If a person likes sex, you know, two people having sex. Quickest method is to bypass the brain and just describe.
If the person has indulged in a lot of Pavlovian conditioning you must negotiate the brain to get to the groin--you might need to describe them being nursed by a large woman whilst wearing soiled nappies.
If a person has indulged in layers of operant and pavlovian conditioning you may need to describe a man being forceable dressed up as a woman and displayed in public against their will in order to get aroused *and* make the work something that could be shelved in ther literature section at Dymocks.
Oh, it is so tiring to involved the brain with its multilayered, imperfect and accute awareness of judgemental observers. A well-defined kink and a discreet ebook, that's a simple way of doing it.