I'm also re-reading my X-Men comics and got to this one a few days ago. I was disgusted and confused by this as well. I don't know why I didn't remember it since I've read Legion Quest/AoA about a dozen times. The real Xavier heard her mind shout for help and he arrived on the scene with Legion looking like Legion and his mom fully clothed. I just tell myself that Legion stopped himself or Xavier got there before anything more than a kiss happened. Still awful though.