While I have no specific source for VR, it's been kind of a meme for a long time that porn has been one of the major driving forces in the tech world and for tech adoption [0]
I guess the nature of the content makes it difficult to actually surface that because only a minority of people are open/vocal about their porn consumption habits or in what ways they are consuming it.
Case in point: Most people have no issue letting their online-social circle know that games they are playing, like in the way Steam surfaces it to friends.
While at the same time there is some demand by people to hide the more "embarrasing" games from their library they don't want others to know they have in their library [1]
On a personal note: I have to agree with the parent comment that porn is actually the kind of content that makes the advantages of VR really shine on the immersive end, particularly when coupled with 3D audio like that of the Index.
Fitness games are cool too, but they suffer from usually being very fictionalized in setting and art style. So while they take you to a place, you inherently know that place is fake.
With porn VR content its usually a real setting, with real people, and you are suddenly right in the middle of it, which adds a lot to the feeling of being immersed in an actual place, and not just some virtual space made up of virtual polygons.
[0] http://edition.cnn.com/2010/TECH/04/23/porn.technology/index...
[1] https://www.reddit.com/r/Steam/comments/9wvfbw/a_way_to_hide...