A Small Pet Peeve

I don't know why it always bugs me when I hear a spouse call their mother-in-law "mom" or father-in-law "dad". I wouldn't feel comfortable calling my mother-in-law "mom" because I feel that the word "mom" is very special and should only be mean't for my mom. It would also bother me if my sister-in-law called my parents "mom" or "dad". Maybe I am just being possessive but that is how it makes me feel.