I think it's weird that within hentai there are a few very specific niches that seem to feature circumcised men. First of all, you have the artwork of Kami Tora
, an illustrator that I assume is from the West because of the circumcision style featured in his artwork (several examples of which show up at the top of the Google Image search that you mention). I include his art as a category because he is in my opinion the only hentai artist that exclusively draws distinctly circumcised penises (and even has a tutorial for how to draw them). I like Kami Tora's heterosexual art a lot, but his catalog is largely devoted to futanari artwork, the category that probably makes up the largest proportion of hentai on the internet that features circumcised penises (even more than hentai featuring straight couples). I consider myself an extremely open-minded individual, and futanari art doesn't offend me or anything. That said, the concept of the beautiful woman with a penis does nothing to get me off or anything. I’m genuinely kind of astounded by the popularity of the genre.
The next category of hentai that commonly features circumcised penises is "furry"-themed artwork. Again, not something that offends me, but not something that I personally understand as erotic. If this does happen to be your thing (hey, no judgement here), I would suggest starting at rule34.xxx
(FYI, if you aren't familiar with the term, Rule 34 is roughly that 'if you can imagine it, there is porn somewhere on the internet of it'), and is most often applied to bizarre hardcore mashups of childhood cartoon characters.
Finally, it's probably worth noting that there's something lost in translation when it comes to the Japanese term for "circumcised." The closest thing to a literal translation would be "katsurei o okanau," or (by my very rough understanding) "to do the cutting ritual." That said, this term is virtually never used because of the minuscule number of Japanese Jews and Muslims for whom the phrase would apply. Instead, the more common term is "hokei no shujutsu": "operation on a phimotic penis”. Apparently, there is some belief within Japan that there is an abnormally high prevalence of men with the combination of a small glans and long foreskin; leading to a large number of phimotic ("hokei") men. This being regarded as undesirable among women, cosmetic circumcisions have purportedly been on the rise as of late. Lastly, a third term, "mukeru" ("to roll back") is used to distinguish non-phimotic men from those that are "hokei." This last term and its variants may also idiomatically be used to describe circumcised men as well, and the term "muke" is probably the closest term the Japanese have to "cut." All of this is perhaps a long-winded way of saying that even when you a Japanese word translated to "circumcised," it very rarely seems to actually indicate complete foreskin removal. My suspicion, based on the contexts that I have seen, is that it is most frequently an improper translation of "rolled back."
For what it's worth, most of the information from the last paragraph comes from Genaro Castro=Vazquez's Male Circumcision in Japan