I wish I couldn't answer this question but I can. Here are the passages I've heard used to support that idea that a transgender person transitioning is sinful. Please note that I do not agree that this is a correct Biblical understanding. First in Genesis humans are created in the God's image, male and female. Thus some argue that male and female are the only options scripture considers. This is fixed and unchangeable the argument goes and it is against the creation order (pre-fall) so sinful to reject the gender one is born into (again not my argument).

Also, not a passage exactly but the argument that trans people are falling into a body soul duality heresy by considering the body (one's genitals in particular) as separate from one's soul (one's gender).