I've been using Itch for several years. It's true that games with sexual content have always existed, but I hypothesize that when other sites started banning this type of content, while Itch has never prohibited it (despite a couple of instances where they had issues), Itch has become a place where people can publish and play this content with a certain level of confidence. This obviously attracts more creators and players looking for that kind of content, resulting in a sizable community within the rest of the Itch user base.
Edit:
With the problem of indexing paid games that contain NSFW content, I don't know what will happen in the next year, but lately I've seen quite a few games publishing free content or demos, and in some posts, some creators have hinted that publishing this type of content on Itch still gives them a lot of exposure.