OK, perhaps it gets repetitious, but I almost always answer this (as Jim has) by direct reference to what a woman
would or should do with the corresponding situation - hygiene of her vulva. This is an extremely apt analogy as for starters, the question really is
the same in terms of the physiology of the areas - not only are they "kissing cousins" during sex, but they are basically the same embryologically - that is, as they originally develop prior to birth.
Viewed from that perspective, it is notable that women do
commonly have problems in this area, and that the "answers" are well understood, though the existence of such problem still indicates that many people are not
aware of appropriate hygiene.
In general, women learn - eventually - that soap or anything resembling it - is the enemy. "Feminine deodorants" have come and gone - well, they still market them furiously, but on the basis that "there's a sucker born every minute" and they supplement the income of gynaecologists to a degree. An astute doctor will inquire about and deal with such things early in the course of investigation of "vaginal" (vulval) irritation.
The other aspect of course, is the seemingly different expectations that men and women have, of the genitalia of the opposite gender. In general, women are supposed to be "pickier", demanding absolute "cleanliness" if they are to offer oral sex to a fellow, whereas men would appear to be far less critical in the converse situation, arguably reflecting the degree of motivation in each case.
In reality, I would personally consider it very reasonable to expect a shower as a prelude to oral sex (and preferably even for "only" vaginal), but many would doubt whether the lack of same is any reason not to enjoy any of these activities, and note that on the one hand the natural lubrication that is the result of sexual anticipation is quite effective in cleaning these areas, while such sweatiness as may remain is in fact harmless, and vanishes promptly on commencement of such activity.
And of course, people should certainly take note that such superficial considerations of "hygiene" have little or no relevance at all to the presence or infectivity of sexually-transmitted diseases.
When someone says to me "Some people advocate ...", I always answer - just who
are these people and is there any
reason to suppose them to have competence in the area in question? Might they be ... Marketers? Politicians? Bloggers on the Internet?