Biff's actions at the dance in Back to the Future were significantly affected by Marty's presence in 1955. Being repeatedly humiliated, including having $300 worth of damage done to his car when it was filled with manure, left him really, really angry. Presumably, it was because of his particularly nasty attitude that night that he ultimately assaulted Lorraine. And remember, he didn't show up at the parked car looking exclusively or even primarily for Lorraine; he was mostly there to get Marty. Lorraine happened to be there, and moreover, raping Lorraine would have been, in Biff's mind, a further blow to Marty; Biff felt that he was getting Marty's girl.
In the original timeline Biff certainly had the hots for Lorraine, at least initially. But without the events with Marty, he would not have been nearly as aggressive in an attempt to woo her. There would be no particular reason for him to go after her specifically at the Enchantment Under the Sea dance. And after the dance, she would be George's girlfriend. Biff would not get a thrill out of stealing George's girl; George was so pathetic that Biff might be turned off a girl who had fallen for George and made out with George.
In the alternate timeline in the second movie, Biff holds a grudge against George and still wants Lorraine. But that is part of the revised timeline, in which George had laid him out with a single punch back in 1955. Vengeance for that (and whatever other perceived humiliations a much stronger George had inflicted on Biff in the intervening years), along with Biff's vast wealth in the alternate timeline, were responsible for Biff murdering George in that world.