Says who?

Okay so I'm going to jump right into this!
Why does the middle finger mean F U? Seriously. Who in the world decided that? Did he just get really mad one day, scream that, and put up his middle finger? Seriously.
Also, why do we have "cuss" words. Me and my mom got into a huge debate over this in our car the other day. I brought it up and she said, "Well, words have meanings,"
Touche, but if they are so bad then why can we use alternatives that mean the same thing? If they have the same meaning then what does it really matter what you say? For example:

Me: What the fuck?
Mom: WHATCH YOUR MOUTH!!
-or-
me: what the freak?
Mom: idk.

Me: They fucked
Mom: Young lady!
-or-
Me: They had sex!
Mom: that isn't someone to hang out with.

you can say crap or poop but not shit
you can say Hell as a place but if you say what the hell it's a cuss word?
you can say ass because it can mean donkey even if you obviously don't mean it like you're an ass, obviously we don't mean you're a donkey that's silly but it's still okay. But you cannot say ass hole, even those its a body part?!?
you can say dick or vagina even thought they are body parts?!?
damn you cant say but you can say darn or dang?

Now, would you all like to know why i think this is happening? Children in America are over protected. Most kids aren't taught sex education and they are left to find it out on their own. Obviously no one realizes that this causes premarital sex and everything they are taught not to do. Seriously! This went from griping about cuss words to sex education? wow....lol.