Pretty much every girl that know of the existence of BJ and actually love (or really want something from) the guy would do that pretty soon. Well, character play quite a role in it, but mostly in the "I dont have the experience" department giving anxiety of underperformance or initial disgust, thats it. I really doubt that the girls dont know about BJ (maybe only Sana, but there is no events with her so far), lack experience (practice on vegetables or learned from the net) or have huge disgust.
Though I am still quite a way away from the completion of the rework (currently just got 1st happy event, and damn it was awesome), so, I dont know, maybe it is logical afterall.