Even when NTR is ON - you have the opportunity to reject all NPC advances.
I got curious and after finishing it with NTR off, I made a speed run with NTR ON, so, here are few differences (at least that I noticed):
1) on NTR off path - LIs will automatically reject all NPCs (there will be still choices , but only with one option - reject), when NTR is ON - there will be choices, only this time - 2 options: reject or accept advances
2) on NTR on path you will see some naughty thoughts from Lara, while on NTR off path she is more devoted to MC and don't have dirty thoughts, at least we don't see them
Long story short even on NTR ON route nothing happens if you don't want to.
Also I never made Lara jealous, there a few scenes when you can do that, so I don't know what happens if you piss her off, though I think you still will be in control.