Why does someone "pull my leg"?

To "pull one's leg", as a saying, does seem to have the etymology you describe; every source I can find states that it dates back to the mid-1800s in England, and refers to physically tripping up another person, which puts him off balance, possibly makes him collide with others in awkward ways, and generally makes him look foolish. It quickly evolved to mean achieving that result - making a person look foolish - regardless of the specific means used. The most popular means to do so is to tell a deliberate plausible non-truth which, if believed, would lead the person react foolishly.

"Pulling one's plonker" by contrast seems to be a much newer term, still considered slang and rare in American usage (it's most common in British and Australian vernacular). It's one of many examples of introducing a sexual connotation to otherwise "innocent" idioms and sayings.

In Genesis 25, Rebekah is giving birth to Esau and Jacob. As Esau is being born, Jacob reaches out and grabs Esau's leg and holds on, thus he is called Jacob, "one who pulls the leg".

Throughout the rest of his life Jacob is known for his deception, trickery and lies to gain advantage over his brother and other people as well. I believe the saying is in reference to Jacob and Esau.