Why do other people nearly always assume my parents must be divorced , when I say I have a stepfather?

My mother remarried after she was widowed. Some people think it's disrespectful to my step-dad, to refer to him as such, or call him by his first name rather than 'Dad'. I think it would be disrespectful to my late father to call another man 'Dad', as I was old enough to know a person can only... show more My mother remarried after she was widowed. Some people think it's disrespectful to my step-dad, to refer to him as such, or call him by his first name rather than 'Dad'. I think it would be disrespectful to my late father to call another man 'Dad', as I was old enough to know a person can only have one father. My step dad knows I respect him, as my mother's second husband.
Update: Thank you to everyone who replied. My mother and step dad have been married for 39 years now. Calling him 'my step dad' and people asking why, is not a problem,I got used to it years ago, as did my siblings. Our children call him granddad though, in my daughter's case he is the only one she has... show more Thank you to everyone who replied. My mother and step dad have been married for 39 years now. Calling him 'my step dad' and people asking why, is not a problem,I got used to it years ago, as did my siblings. Our children call him granddad though, in my daughter's case he is the only one she has known, as my f-i-l died before I got pregnant. I could never have called my in-laws Mum and Dad either, as my parents were expected to do with theirs. [Not that my inlaws would have liked anyone but their children call them Mum and Dad.
I was just curious to find what people thought.
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