There is no story reason for it. Halsin was never intended to be a companion, hence "the wall" in act 1 and 2.
But some people thirsted constantly for Halsin sex during EA that Larian decided that they could score easy points, especially as sex became the main selling point of the game when everything else failed.
So the story was rewritten and Halsin became a companion in act 3 (he was supposed to stay in act 2) and as only character trait they gave him "polygamy bear". But Larian did not rewrite the old and already finished content where Halsin is not sex crazed.
Pretty much. Personally (especially as a small halfing who had no interest in Halson) I found his sudden shift from being pollite professional, to sex addict rather unsettling.
