As a pansexual in a straight relationship, I understand the whole bisexual erasure thing and I think it would help to:
1) acknowledge bisexuality, and be open minded about people you don't know and believe what they tell you
2) Recognize a difference between an individuals identity and the identity of a relationship. Bisexual people can be in straight or gay relationships, straight people can be in queer relationships (in the sense of gender unconventional), you could even be in a relationship that seems incompatible with your sexuality, theres a long history of gay people marrying the opposite sex. And yeah that might be because of.. complicated reasons but a relationship is still a relationship. So many people don't even have romantically traditional relationships.
But also... I think this couple needs to have a serious talk about their relationship, because it sounds like either kate is still deep down recognizing andy as a woman in her lesbian attraction to her, in which case on some level, conscious or not, she's in denial that a gender transition is truly a transition... and I can only see it causing problems for their relationship in the future. Or she's bi/pansexual in which case her demand for the minority cool-points of non-straight-passing just seems unreasonable to me. Tons of bisexual people are in straight relationships. It sucks when people deny you when you claim that you're bisexual, but if your effortlessly fitting in at lesbian bars means more to you than your partners comfort it just starts to seem like the same as straight people who want their own pride parade. Your partner's trans identity shouldn't be your badge to wear to escape the terrible curse of untrendy heterosexuality.
Personally I didn't specifically choose a straight relationship for any reason, it doesn't make me less pansexual that I didn't care what gender my partner was when I fell in love and if people want to insist i'm 100% straight do I actually have any reason to care?? Ever? And more importantly, it's NOT my partner's problem.