Hes not his fated pair... he has Bailang Blood that makes it more confuse who is the real fated pair
Xiaobei is danqing fated mate. Even he drinks bailang blood, it won't change anything unless bailang give him permanent mark (by sex).
Agree.. he steal his brother's wife by marking him permanently without consent. From what it looks bailang trying to have sex while xiaobei is unconscious (permanent mark).
And the important thing you mentioned, he didn't ask XiaoBei his consent of he wanting to marked or not, XiaoBei would rather die than lose his freedom, that's just make XiaoBei hates him more if he woke up.
Bailang just fuck him while he is unconscious? I mean he is Dangqing's fate pair so that's stealing your brother's wife... I think once Dangqing find out he will drag Bailang to hell...