I think that with all the burden of social conditioning around gender and body-shape and genitalia, and with the immense complexity of people's sexuality and orientation on top of that, it's a bit harsh to castigate someone for having a different level of attraction towards a partner depending on what genitalia they end up having.
People fall for a particular person in a particular shape, and when that changes, it does change things - and that's either okay in the end, or it isn't. People go off each other for things as apparently trivial as a change of hairstyle, so I think it's naive to say "If you loved me as me, you wouldn't mind what parts I have..." - if you're going to change your body, you have to expect that to rock the boat in a relationship, even if in the end no-one abandons ship. Either way, it's wise to be prepared for turbulence.