I may be incorrect about this, but in my experience he will start asking if he can fuck you after watching porn with his friends if you're already in a sexual relationship with him. I've never looked at that segment of code myself so I don't know for certain, but that's been my experience. Kolka may also start thinking you're a slut, but I think that happens even if you are exclusive to him (again, anecdotal evidence only, I have not looked at the internals of most paths I personally am not interested in).