The thing about stages and phases is that they pass. So, ultimately, if it isn't, time will tell your Mom the tale.
In other words, just give it some time, rather than trying to convince her she's wrong. After all, neither you NOR she can predict the future.
Really, sexuality is fluid, so even it it WAS a stage or phase, that shouldn't make a difference in anyone's acceptance. After all, given our culture thinks of heterosexuality as a default, one could say that everyone bisexual or homosexual had a heterosexual "phase," eh? Many homosexual people absolutely had a bisexual "stage," too.
So, if you do want to talk about it more, one approach you could take is to just ask for acceptance even if it DOES turn out to be a "phase." But otherwise, I'd just let time do the job for you.