I believe Amy did not have alternating preferences, she was a lesbian. If your father writes a multi-page letter to your love based on something not a choice, but your true self, not only is it identity crushing, but if it's also been a continuing unspoken vibe, what better reason to start a 'relationship' with a man. Squash the judgment, keep the peace, do whatever to shut everyone up until you...have a new apartment, the job, new dog, fresh start to live your own life. This sounds familiar.
Pure speculation because we will never know for certain: with so much togetherness/family time on this cruise, I'd bet on several 'deeper' conversations (potentially one-way at times), including the one with Brad on the balcony. They were undoubtedly close, so maybe she valued his opinion over others, or he said something triggering (not necessarily intentionally). Amy's life is changing, and it sounds like a reconciliation was happening with letter in a bottle gf. I'd like to see the rest of the letter, but it's for the gf. And "Save Me" seems compelling enough, even figuratively.
I don't know about flat-out disowning, but maybe Amy realized her life would never be considered 'ok' with her family, and because they were close cutting off ties was not in the caards. An outright come to Jesus talk would be a very dark cherry on top, but it's also possible this was just in her head and tormenting her none the less. She's sad, confused, and potentially not seeing an enjoyable future while sitting on a balcony with some heavy existential thoughts. We know she used alcohol to self-medicate, maybe she had more or other pharmaceuticals with her. It very well could have been an impulsive, fogged decision to jump, it just happened to also be final.