You may not care, you may call it stupid. But inside your thinking "wtf???? weird..."
Back in the early 1800's when a man had an erection they called it a horned man. This led to slang being formed and the word horny was used when a man had an erection. Later on, people called anybody who was sexually excited "Horny" whether it was man or woman.
And THAT is how horny originated. Don't call me a perv for knowing this, I just know a ton of stuff... Useless stuff, but stuff nonetheless.