if you have your mom to introduce you as her son and your girlfriend who has always said 'my boyfriend' ... then is there a reason that her mom will doubt that you are a boy? the most she might think is 'what a cute boy' in her head.
does your girlfriend plan on telling her mom/think her mom will be ok with it like your mom is?
i am dating an ftm (1 year, 7 mos) and i do not ever plan on telling my parents that my boyfriend is ftm. not because i don't think they can handle it, just because i don't think it's that important. the most i might ever say is that because of a birth defect, he can't father children... but maybe only that if we got married.
i know that this phrase often brings negative reactions, but in some situations 'don't ask, don't tell' really *is* the best option