Saying this will make people, including any trans friends you have, go "hmm" at you if you're not careful.

There are probably cisgender people with this as a kink, but the concept is almost exclusively promoted by people who fear trans women and think transitioning is all about sexual gratification. Which is complete nonsense.

Besides...it's just another kind of transformation fantasy even for those hypothetical cisgender people. It's harmless. It's not a dysfunction unless you're some kind of weird prude who thinks sexual fantasies are dysfunctional. Go off and imagine yourself as some fantastical creature being gently, consensually railed by a dragon. It's fun.