Why does the Bible contain impolite phrases?

Answer:

Terms that are concerned polite or acceptable change between cultures and between time periods. The words used just a hundred years ago to refer to people with dark skin coloration are now considered so derogatory today that a politician using such words would be marched out of office. I'm sure some American expressions, such as "kicking a dead horse," might sound offensive if literally translated into another language and culture.

The phrase used in the verses you listed is mashtin beqir in the Hebrew. It literally means "one urinating on a wall." It is an idiom for a male because males are able to do this while females cannot. In particular, it is a trait of male dogs as well as human males. The King James translators wrote the idiom as worded into English, but you will find that almost all modern versions will translate it as "man" or "male."

When David said, "So and more also do God unto the enemies of David, if I leave of all that pertain to him by the morning light any that pisseth against the wall" (I Samuel 25:22 - KJV), the phrasing is not profanity. He is saying that he will not leave one male, even down to the dogs, alive by morning. Some translators suggested saying "if I leave him even one dog by dawn," but this doesn't capture what is being said very well since dogs can be male and female and the focus is actually on humans and not animals. Some others suggest "if I leave him one man by dawn," but this could be seen as leaving out the animals and the boys. Most say something like, "if I leave one male of all who belong to him by morning light." This captures the broadness of the phrase without sounding offensive in modern-day English. You might miss the completeness of what David is saying, that is that he won't even leave one male dog alive, but it retains the focus on the humans with whom he is angry.