Nice girls get angry when....
Do you think Marty was ACTUALLY LITERALLY planning on sexually assaulting Lorraine for George's set-up? It certainly sounds like that was the plan....
This makes the whole incest aspect even darker. Lorraine was completely unaware she is his mother, so from her point of view is excusable.
However, Marty is painfully aware that she is his mom.
Was he really willing to go to such lengths, sexually assaulting his own mom for the benefit of the plan?