I think it has to do with cultural norms on what is acceptable to show publicly vs. in your private life. Video games are considered "public" by many due to the wide audience, despite the inherently non-social nature of singleplayer. This often combines with a weak understanding of gameplay mechanics and strong opinions about what should be accessible to certain audiences; producing purists on the issue.
There's also a pretty widespread opinion, even among gamers, that games trivialize any subject they include. Violence in video games is almost as old as video games themselves and was already 'there' when they started becoming mainstream entertainment. Sex, with some exceptions, is reasonably new as a "feature" among games that are not designed with it as the primary goal or reward (*shudder*). Ergo, the few mainstream games (and yes, I use the term loosely in AP's case) that allude to such things are easy targets for crusaders.