There is nothing wrong with being gay. It is seen all throughout the animal kingdom. Being trans though, that is seen only in humans. Therefore, it is not natural, but self imposed.
however
score: 17
Why does it matter if something is considered natural?
Gender dysphoria is a very real and diagnosable disorder, therefore it occurs "naturally" and is not socially imposed. It doesn't matter if it only is detected in humans, it doesn't make it wrong. Source below.
Why could animals not feel in similar ways? We cannot ask them how they feel on the matter, so for all we know it's common. The only real difference is that we (and several species of fish) have the ability to act on it.
Being homosexual has nothing to do with what gender you feel you are. A trans-woman (post operation) who is sexually attracted to women used to be a heterosexual male and "became" a homosexual female.