Realistically, your best choice is to just 'be more mindful in the future whenever the Nameless is involved'.
Your game won't be ruined by your first decision, and your particular herbalist can be just as astonished as you are.
Yeah I chose 'serve Nameless' to test it out... welp, guess my attempt with different savefiles didn't work...
So far the only event where I got a taste of it was at the Plains of Death. I do wonder though, if there was no sex scene at all with Nameless if you didn't choose to serve him back at your home..?