Is it normal to write masturbation?

My mother always told me it was really bad to spell masturbate as 'masterbate' and that I should never do it.

My teachers always told me 'masterbation' was wrong.

My priest always told me that 'masturbation' was OK though.

So many people here write 'masterbate' and I feel like such an outsider for writing in correct English. Is this normal?