Regarding the argument over lolis in this thread:
Getting back on topic, other than the obvious sentiment most people in this thread have agreed with regarding freedom of derivative content, there's a few more factors that I personally believe have played into this.
First of all, the data regarding NSFW Touhou art in general (as the OP only breaks it down by character) is roughly as follows:
4% of galleries submitted to e-hentai have at least some art featuring Touhou characters (not a very good representation as a gallery will count here even if there's only one or two images with Touhou characters in them)
3% of all images with an explicit rating submitted to Gelbooru are tagged with "touhou"
1.4% of all images submitted to Rule34 are tagged with "touhou"
While the data on this isn't quite clear, compared to the total number of Pixiv submissions, Touhou accounts for 2.4% and Touhou works rated R18 account for 0.2%; do keep in mind that this also takes into account non-NSFW art as there's no data on how many works submitted to Pixiv are rated R18
These are still pretty big numbers when looking at it comparatively, but maybe not as big as someone might think so I thought it would be good to express this data here.
But anyways, the fact is that the Japanese cultural framework around sex and sexual expression is so radically different not just from most western countries, but from most countries period. I'm sure many people here have seen the "It's called hentai, and it's art" meme at some point, but the thing about that is that from a Japanese point of view, there's a lot of truth to that statement. Now, obviously, being a sex pest should be enough to get you ostracized from just about any social circle, but merely creating pornographic depictions of fictional characters is (at the very least within otaku circles, where Touhou is most prominent) typically not seen as any different to just creating regular derivative work. Sadly I'm not nearly educated enough to accurately explain the social and historical factors that contribute this, so this surface level analysis will have to do.
Another thing is that the number of male Comiket attendees (no data for Reitaisai but intuition leads me to believe that this should be the case there as well if it already wasn't before) has been steadily increasing since the dawn of the century, with men potentially even making up the majority as of a few years ago (or at the very least it being somewhat even compared to the previous overwhelming female majority). And the thing about Touhou is that it has at least one female character that appeals to any kind of moe you can think of. This then encourages doujin artists who focus on making doujins for men (i.e. mostly other men, and those doujins mostly focus on the female characters so as to allow the male reader to self-insert) to enter the Touhou space because not only does it provide you with already finished characters that appeal to whatever archetype you want, thus sparing you the need to come up with an original setting or plot, but is also very immediately recognizable. This also means you'll likely make more money, as you don't have to worry about competition too much due to most people sticking exclusively to hentai works featuring only a few select characters that they like (though that's a secondary factor I'd assume since doujin culture is never about the money first). And all of this not even considering the guys who already like Touhou and are making doujins of it right out of the gate.
Though all in all I personally see no issue regarding any of this, if anyone's actually upset by the mere fact that NSFW fan content exists (which is like, come on man you're on the internet) they can always just, y'know, not look at it. I will say one thing though, and that's that some of the most entertaining and thought-provoking media I've experienced falls into this category, be it erotic doujin manga or visual novels with sex scenes or anything in between, so it's definitely not worth avoiding something just because of "oh no sex".