I am not sure if there is that scene if MC was cold towards her but I played the nice guy path and I think it felt there was good enough built up tbh. Even after restaurant scene was good, though she is feeling sad prevented it. So, it was not out of place for me or how it happened out of character for her. Though, I do understand why you guys feel that way.
Ah, now I get it. You need to divide that scene into 2 pieces. Bj part and tying part. Tying part comes from her vulnerability and fear of men. She is broken and unless she has the feeling she is secure, she is not going to do anything. Other part was bj, their relationship reached to that point in my opinion. Merging these two was pure coincidence though. She has seen MC's dick, she has the opportunity to satisfy her curiosity. So it was spur of a moment thing. On top of that, she is totally into MC and MC almost died for her just before that scene. If MC tease her about it afterwards, she becomes her kinda prudish, conservative persona and stops MC.
I really don't see a problem with it but it just my opinion.