i think rian is putting up with it because he's trying to uncover the real umbra project and stay alive. because enzo literally is the threat to his life, he's trying to avoid being disposed of by being useful to him and simultaneously trying to dig up useful information in hopes of getting some leverage? either way both require enzo's favour so that's why he's enduring it, plus we all know he'll end up falling for enzo eventually. and enzo is just mentally disturbed and violently possessive, it doesn't help that rian's main asset is his dick, hence the "submission" which is obviously rape but it's not disguised as affection it's clearly a power play.
i still think the author is doing a good job basically, nothing about this is out of character or ignores the context.
Ive been kinda defending the author on this because it seems they have a clear plot in mind, it just requires an absolute jerk and a very uncomfortable action along the way. As long as they know what they're doing, i think i can accept the black flag. I mean look at my pfp, that's how much i love this manhwa lol. However the plot seems lost to me now i don't even know why they're having sex... is uno held captive? Why didnt he try to run away anymore? Does the author really just throw away everything and expect us to be 'okay' with the rape just because uno is being nonchalant with it? Calling it 'submission' doesnt even make it cringe, it makes the whole thing much more sad and devastating, because that's just rape isn't it? Make uno cry and it'll make more sense. Now it just seems like an excuse to dial down our rage. Is it rape? But he's not in so much pain, don't deny it when you're this hard, if you don't want it then say it why don't you—kinda sitution... im still here i guess, but with huge disappointment and very faded hope