The main issue with VR is the headset and its support.
If you dont want facebook, there are no alternatives that are cheap. And there is a good reason to avoid facebook. Im not going to spend 1000 euros for something im barely going to use (purely for porn is not enough to justify that price). Allowing any phone with a gyroscope would be ideal here.
And with that i would just use a player on the phone and have that synched with MPC-HC, MPV or whatever video player you use on the pc. But support here is almost non existing. Sure, if you have a samsung phone (which not everyone has, especialy since the VR capable ones are generaly expensive), you can somewhat work around it, but even there, synchronization is limited.
And for VR players on phones, spending money without knowing if it works is a bad idea. They might play VR fine, but without synchronization, its not usefull. Sure, some might also have the content included in the player, and have decent synching. But again, you have to pay money for trying that to begin with. Having some sort of trial videos to at least test functionality would be welcome (and they dont even need to be porn videos, just a good indication to check if the VR part works and a good indication of device movement etc would be enough for a video).
And yes, some phones might not have good detection for moving your head. But usualy that doesnt realy matter. a 360 (or 180) degrees view is often more than enough. Its at least a good step for people to get into VR. Even static SBS videos without rotation can already make a diffirence just because you see depth.
But even using a 3rd party platform isnt enough when you want to script for VR yourself. In that case you just cant get around a direct connection with your pc.
(also, from my experience, if you set the video to a single eye, you end up with a fisheye effect, but zooming in does resolve that quite decently. A lot of VR videos are basicly depending on that very similar window so small distortions are often not going to be an issue. But again, limited support for these things makes most players unable to handle this)