I don’t think so, personally. They can change their outward appearance but they can’t change biology, even if the outward appearance changes. If you guys ever broke up and you found yourself attracted to men, then sure you’d probably at least be bisexual. But just because she decides to become trans does not inherently make you gay if you are still attracted to them. You guys are bonded. That doesn’t just go away because she decided to become a he.