Torluf(and us by choosing his route) breaks Arvo on a fundamental level. We choose that Arvo "feels comfortable" with Torluf's flirting, but based on the little knowledge that we know ABOUT Arvo, his personality, and past, would he ACTUALLY be okay with it?
If we step out of our "We control the main character" point of view, and look at the situation through, say Lake's or Mikko's perspective, we would probably see some stranger changing your friend from Mikko's view, and a person who you seek validation from(Via compliments, contact, or sex,) pitting your friend against you from Lake's view.
The bathroom scene after the third NSFW scene shows Arvo(almost) realizing what's happening, and getting disgusted by himself and his actions(At least, that's how I interpret it.) He also confides in the ram lady who's name I forgot about it, and seeks advice from her.
It's also important to note that this doesn't happen in ANY other routes. Not after anything with Bjorn, not after anything with Rune, not after anything with Jorgen. It's only in TORLUF'S route. I may be copying what someone has already said, but the other routes bring out different parts of Arvo's personality, while Torluf's route changes Arvo into doing things that he probably wouldn't do on any other route.