They are mutually exclusive and the trigger is whether you had sex with Axel in his apt., during the photo shoot, or not.
The big difference here is that If you had sex with Axel before, you will basically get your face raped, no matter what you do, you can´t say no. While with Jack you can still decide if you want to or not.
Edit: Doesn´t matter if you are in relationship or not, you can still trigger the scene, same as above, except there´s an extra step where you literally have to choose "cheat on ian" with Jack.