When it comes to one's own life, one isn't always the one's best judge of it. Lois sees her life through her own Lois-hazed glasses. We like to say here on the boards that Lois isn't an honest narrator (i.e. if the story is from her POV, then maybe the facts as she tells it aren't exactly what she says they are.)
Her father doesn't care, because that's how she always saw him, how their relationship felt, (and Ellen probably reinforced this belief... as often happens with parents of divorce. "He not only cheated on me, he cheated on you two girls, too.") It also could be that now that Sam's older, he realizes that he has missed out on much of Lois's life and wants to make up for it... but she isn't willing to give him an inch, because of how strict he was with her earlier.
With Paul, both could be true. It could be that Lois and Paul had a relationship, but then he dumped her for Linda. It also could be true that Lois THOUGHT they had a relationship, but he was taking it casual until Linda, when he no longer wanted Lois. Lois could've WANTED a relationship with Paul (daydreamed of one) and when he started dating Linda, it felt as if Linda had stolen him. Therefore, when she tells Clark that Paul was her boyfriend, she's telling him how she felt about Paul (whether it's true or not), but at the end she admits to Linda (and to herself) that there really hadn't been a relationship there.
Usually, most writers have Paul being the jerk because Lois says most of her relationships were federal disasters, so we lump Paul into that group. But feel free to re-write history and have it happen another way.
