People choose to believe or not in things based on one thing: what it ****ing suits them.
Your ex is not even the worst example. You can have people who claim to be perfectly fine with sexuality, even say god is also fine with it, but when faced with a real life decision always back out. I'm really sorry you've gone through this, it sounds awful. It's so heart-breaking that a genuinely good relationship that people so badly search for, and may never find, can be thrown away by someone who doesn't appreciate what they have. Most people in that situation would surely choose to keep their relationship and work around Christianity, as many gay Christians attest to; again, because it suits them.
Clearly some very strong scarring, persuasion, brain-washing or whatever you want to call it took place with your ex in order for the identity to be irreconcilable linked to a tight interpretation of the Bible, or even a wrong interpretation spread by so many people. They've made their choice. There's nothing you can do but have the dignity to not beg them to stay. Just walk away. You shouldn't want to be with someone who doesn't value you.