One of the most common problems I've seen in stories: BIOLOGY is totally disregarded, with people having no knowledge of how humans reproduce and how EXACTLY sex happens. Also, they tend not to know the various organs in the body, including the fact that males and females have different sex organs for a purpose? 

And maybe they just casually tend to forget that you CAN'T have sex once and BOOM! You have a baby. No. And maybe make them use protection because SEXUALLY TRANSMITTED DISEASES EXIST folks!

But don't worry, we'll cover all that!