Only in Ovid's version of the Medusa myth(one of the last of the classical versions ever written) was the rape by Poseidon added. Something unlikely as Phorcys and Ceto thee parents of Medusa were also the Gods of the sea, in an entirely different pantheon to Poseidon.
In most versions of the story the Gorgons (a Greek word for Grim ones) were born monstrous and only Medusa has snake hair which was considered a blessing as it made her an even more fierce warrior.
She was slain by Perseus because he was commanded to do so by Polydectes a suitor for his mother as a wedding gift to them. Polydectes did this not because he hated Medusa, but because he believed she was the only warrior capable of Killing Perseus. He was wrong of course, Perseus being the bastard son of Zeus was equipped with magical weapons in order to cheat (He killed her while she was a sleep).
Right up until Roman times the image of Medusa was worn on breast plates of great warriors as a sign of respect and power.

I had taken that Medusa was a prude and was banished because she was castrating men making them agamic.

Great post