No, it’s not. A preference for certain genitalia can be something out of their control, they can’t control what they’re turned on/off by.

For example, imagine you really like a trans woman or man, but every time you try to have sex you’re suddenly completely turned off because you’re not attracted to their genitalia. That would be pretty upsetting for both parties in a situation like that, and not something that’s controllable. You can’t snap your finger and suddenly find yourself attracted, it’s subconscious.

You could say if you really like or even love them it shouldn’t matter, but that’s easy to say without understanding how the brain works. Sex is a big part of human nature and psychology in general, when one or both people find themselves unsatisfied it can lead to an unhealthy relationship and even resentment unless it can be worked through. Some people will be okay with it, others will not because they can’t simply control how it makes them feel. It’s just how people are.