Bruce is an eldritch creature born of Gotham to protect the city. He's able to appear 100% human, so he spends most of his time in his human form and has never shown a lover his real form. He's wanted to, but his real form is deadly to humans due to his razor-sharp nails and teeth and his poisonous skin. One scratch of his nails or one touch of his skin would be all it took. So, he remains in the form of Bruce Wayne, despite the discomfort he feels at being contained in a human body for long periods of time. He's seen enough death in his life; he doesn't want to be the cause of any more.
Then he meets Clark Kent, a man with invulnerable skin who is immune to any earthly poisons, even those from eldritch horrors like Bruce. He can't hurt Clark, and Bruce has never felt such relief.
For the first time in his life, Bruce can be who he really is with a lover without worrying about killing them.