Sorry, what do you mean by this? Do you mean the option for the crystal or the threesome scene?
technically both
when you interact with the crystal once you see the flashback
when you interact with it the second time you get a choice
the option let kay control her gets you to the threesome plus the "warning" that because of this your options with her are limited
if you try the other option to calm her with the serum there are no interactions with her afterwards
so the question is if I missed something to continue on with this path or is this just the end of current content?