How do you think our species could have survived for millions of years if we were NOT disgusted by leeches?
You're just behaving like a natural woman, protecting her flock.
How people behave when they see blood, leeches and things like that, is instinctive.  It's just built into that person.  Not a single thing you can do about it.  
Incidentally, how you behave when you get your hand operated on under a mild sedative is ALSO just how-that-person-is-made.
I recently had surgery on my hand, and was appalled by my getting woozy and sick to my stomach while they were PULLING and YANKING on my tendons and ligaments and I could feel gallons of blood flowing down my arm.
Until I was wheeled out to recovery.  There was a GIGANTIC pro football player out there, clutching his hand and whining, 'Juuuuuuice box.....cooooookie......ohhhhhhhhhhhhh.....' after passing out.
I asked the nurses about it and they gloated excitedly, 'We LOVE to watch all the BIG STRONG MEN come in here and act SOOOOOOO macho, and then PASS OUT!'