I haven been tinkering around the event last month, so it now has extra lines depending on Rowan's and Alexia's choice on what to do with greyhide at the end of the threesome event, and it should now be possible to trigger it with NTR off:
1) You have to select "Have you considered trying men?" in the first event.
2) Rowan has to agree to the sex scene with Greyhide. (otherwise the threesome event won't trigger)
3) Rowan and Alexia have to agree to pursue a relationship with Greyhide at the end of the threesome event.
Also, if Alexia and Rowan decided not to pursue a relationship with Greyhide, Alexia now needs to be on medium corruption to break her promise and willingly cheat on her husband.