It's not exactly true -- in Ch.14 if Ian and Jeremy have unresolved conflict over Jeremy telling Ian that Lena has been cheating on him, Lena can convince Jeremy to fuck her even if Ian isn't on cuck route (it's enough if Lena made decision she'd keep cheating) and Jeremy states explicitly that he doesn't feel like he owes Ian any loyalty (anymore)he won't fuck Lena if Ian is in relationship with her, even if Jeremy and Ian are enemies, he'd only fuck Lena if Ian on the cuck route.
Python:
elif lena_cheating == 2 or ian_cuck > 1:
if lena_cheating == 2:
l "Actually, I don't think Ian will mind if you and I got to... know each other a bit better. As long as we keep it a secret, that is."
j "Well, he's not my friend anymore, so I don't owe him any explanation."
l "Then we're on the same page."