because they are uncomfortable with using the proper names of body parts and for sexual activities.

Just theorizing here: Because the parents are uncomfortable talking about the subject and try to 'prettify' it. Because all too often the child will be given detention is it repeat those correct terms in school.

Good question. I think it has something to do with embarrassment. We're too shy about some things.

Because their parents did. Many people are programmed by their upbringing (often, but not always, influenced by religious beliefs) to see sex as something dirty or, at best, simply something that should never be talked about. Their "private parts" are also treated in a similar fashion. I never heard the word vagina until the 6th grade. The kids in the class thought the word was hysterical. When my son heard the word from his older brother (who had just had a class involving proper biological terms), my ex flipped out. He claimed that 5 yrs. was too young to know the words penis and vagina. I explained that experts believe children should learn these terms from a very early age because, among other things, it helps prevent child sexual abuse and gives children who have been abused the tools to report it clearly (it also helps erase the stigma). He continued on his rant about how so many people would think it was an inappropriate word for a 5 yr. old to know, etc. All I could do was tell him that those people are ignorant. There was no way to break through his programming. To this day, my son struggles with the word penis because of my ex's overreaction. If he has to refer to it for some reason, he says his "front private part". He's 9 now.

Because the proper terms aren't all that great in mixed company and whatever words or phrases your child hears will be repeated at the worst possible moment.

Guess I'm guilty. My daughter has a hoo-hoo. When I gave her 'the talk' I used proper terms though. She was 10 and handled it better than I did.