I don't see how this is "cheap" writing. Many back stories have things of this nature, whether it be murdered parents, abuse, murdered loved ones, growing up poor or with a single parent. There are many more examples of bad things that happen to real people that are used in a characters back story. It doesn't make it cheap because it's rape and you have a problem with it. Also I don't ever remember Serana saying anything about being raped. She implies something embarrassing that she'd rather not talk about which could be a number of things, seeing as Daedric princes have their servants do many things for them (murdering someone with a rusty mace or eating the dead). Also her wiki entry doesn't mention anything of rape either http://elderscrolls.wikia.com/wiki/Serana I understand it's a touchy topic for many people, but I don't see what's wrong with this. You may not like it, which is completely fine, but that doesn't make it off limits for writers. Like you said it is inconsiderate of a certain part of the audience though.