That "greyed out" option requires 2 or fewer hard sub points, if I'm reading it right:
	
	
	
		Code:
	
	
			"Change the subject" if roxy_hard_submission <= 2:
	 
 Prior to this scene, you can pick up hard sub points here, so basically skip these, or only pick up two, I guess:
- Update 5: When Roxy visits you during your lecture and starts blowing you, forcing "hard deep throat" scores +1 hard sub point
 
- Update 8: When you go to the pizzeria and Roxy serves you, choosing "be a rough guy" scores +1 hard sub point
 
- Update 8: When you arrive at Roxy's to find her tied up on the bed, choosing "rough" scores +1 hard sub point
 
- Update 11: When fucking Roxy in the classroom after meeting her and Nikki, she asks you to cum in her mouth, choosing to creampie her instead scores +1 hard sub point
 
- Update 14: When you talk to Roxy after she fights (verbally) the guy in the corridor, choosing "try to push her more" scores +1 hard sub point
 
I'll be honest, I don't use the mod, and I think the extra text there ("Roxy sub path") is just from the mod, so I'm not sure how it's categorising it. There's nothing in the game script that talks about a sub path, and the only mention there is that the option is disabled if you have more than 2 "roxy_hard_submission" points.
Edit: Thinking about this a bit more, what is it that you're trying to achieve 
Marecotroll? When you say "submissive Roxy path", what are you expecting? Certainly from that one choice you screenshot, and not knowing what else the mod labels "sub path", that looks to be what I would call a "normal path". On that path Roxy is in a normal, loving relationship with you, though you still have some kinky sex now and then because that's the sort of person she is. I don't think it's a path where you are particularly dominant and Roxy submissive though, if that's what you're after.
I don't know how far you can go getting all the hard sub points, because I've not really explored that side of it. I remember on my first (blind) playthrough, I went rough on Roxy when she was tied to her bed, and I think I walked out after I was done and left her there. She didn't like that and I think the next time I saw her she kicked me in the balls and shouted a lot, and that was pretty much the last I saw of her on that save