Amusingly, a lot of studies lately have been showing that sex doesn't sell nearly as well as it used to due to a combination of factors; specifically that sex sells short term but if the quality isn't there to back it up than consumers go elsewhere for what they're looking for and that the ease of the internet allowing you to find titillation, porn and more extreme stuff with ease means that people who want that kind of stuff now don't have to put up with a half naked loli showing you her bellybutton in a manga, they can just search "guro loli doujin" in Bing and find 30,000 entries. As a for instance, look at Playboy magazine now and how barely any of you have read or seen an issue of it or can say who Miss. Whatevermonth was this year because you can literally just look at tits whenever you want online.
For something like Kumo it could go either way; if someone sees the hot girl on the cover, they could pick up the book and either: 1) be disappointed there isn't any hot ladies in it and put it down or 2) realize it's actually a good manga and continue to read it for the manga itself. Which means sex may originally have caught the person's attention but what brought them back to the table or kept them going was the actual manga itself.
It's like Yuusha ga Shinda; I'm sure the overload of fan-service catches a lot of people and even people who read it continuously drool over it, but I have serious doubts that so many people would keep reading if the manga itself wasn't funny as hell with surprisingly good plot and characters.
I think mainly sex can bump the short term sales of a piece of media pretty well but will bottom out after a while with only the people it panders to remaining consistent if it doesn't have anything else going for it.
