I know exactly what you mean, I dealt with the same situation a few years ago and ended up getting kicked out. - Here's the thing, some parents don't think that being bisexual is normal, while you and I know it is. But parents want their kids to be normal, perfect even.

Honestly the best thing you can do is say something like, "you know what Mum? I love you but this isn't a phase." If she doesn't believe you she will in time. The joking about having gay children thing is, in my opinion totally uncalled for. If it bothers you, tell her. And not "Mum, I need to talk to you". From experience I know sometimes you need to use a more aggressive approach when you want parents to listen.

Good luck. =]

If I were you, I'd just say "fuck it" and ignore it. However, I'm a bitter person. I suppose you should talk to her, give it one last go, and be completely honest with her and tell her you want to talk to her. If she decides to play that "you're not bi" card again, then she's obviously not willing to talk to you seriously.

I told my mom I was gay about a year and a half ago, give or take. She pulled the whole phase bullshit, and pretended to accept it. Eventually she lost it, couldn't pretend and completely bashed me for it. She ranted about how I'm an asshole for going against the family religion, about what the family is going to think, how I'm not gay, its a phase, and it's a crime against nature. That night, a lot changed. Her view hasn't changed at all.. nor her understanding or willingness to be there for me, but I made sure she was aware how abusive it was to be saying said things to me, and in the current situation how it wasn't helping things.
Now, if it ever comes up that i'm gay, or I'm worried about something involving a guy, she instantly says I'm making a bigger deal than it really is, nobody cares if your gay, etc. She completely avoids it, and tries to defuse any confrontation before it starts.

Its probably unhealthier than what we were at before, but we are on a don't ask don't tell policy. Until it blows up again in my face, it's nice. but that's also me being a distant person and liking it.

Confrontation is what made it so much easier for me, but its also different with bisexuality. if there isn't objective to confront (as in, you aren't conflicted between a guy or girl, or alternatively she leaves it alone) then its probably best to wait til there is a reason.

I say talk to her about it before you leave, so she has some time to think about it without you being right there.

Screw your parents. Who cares if they think it's a phase.