I am not sure if the flags were really bugged as it was stated in the notes for patch 6 that a platonic path was added for him, indicating that there wasn't one before. Previously he just assumed the two of you were on flirting terms, even if you never did flirt with him before. He for example always assumed you had asked him about previous lovers, even if you hadn't - though that might have been a bug.
The whole brothel situation is probably largely to blame for the sentiment that Halsin is sex crazed. Apart from him inserting himself into the orgy, his dialogue during it is also very cringe. Worse, while he insists that you make sure you settle things with your partner during the poly-proposal, he (same as the PC) simply goes along with how the orgy proceeds when the other participant is miserable, which also casts an unfavourable light on him and makes you question how sincere is usual more wholesome demeanour is.
