Why people do the things they do...
I think that MOST of the reasons that people do so many "strange" or "bizarre" things is because society views them as taboo, naughty, and they try to control people into saying that they CAN'T do these things. Be it incestual thoughts or behaviors, strange sexual or social thoughts or behaviors. Society CREATES such strange thoughts and behaviors in humans by attempting to control what we do, what we think or how we feel and interpret things.
This is the reason that VERY few people in other countries who don't share these same cultural controls don't have these thoughts or ideas. So in reality, it is COMPLETELY normal to go against the "norms" of society and to have thoughts or fantasies of something that is "naughty", "taboo" or "not allowed". So the next time you think about asking if something is "normal" or not think about what I have said here. Perhaps what you may be asking about is not considered normal, but it IS normal for you to want to experience what is not "allowed" by society. Hope this helps some :)