I tell people all the time that orientation labels are supposed to help a person define themselves, not the other way around - if a label doesn't fit, it's not the *person* who's wrong. In other words, you're a straight woman who's involved with a woman, and sometimes that just happens. (I know lesbians who date men, gay men who date women, asexual people who have sex sometimes... sometimes people are fluid and hard to contain within a single label.) Your mother is going to have to accept that you know your own life and desires better than she does.
My mother actually told me to choose between my wife and her. I did; I haven't spoken to Mom in several years now. That's not a fair thing to ask, and the person demanding that you choose is the one you should look at the hardest. Perhaps she means well, but attempting to break up your relationship - a relationship that makes you happy - is not the best way to go about this.
