Hell's Demon; Heaven's Angel (BoyxBoy)
Rowan Die is a Höllenengel, a cross breed between vampire and angel.
Rowan lives a truly difficult life. Juggling between being the immortal guardian of Sky Wilson, the blue eyed beauty he's intensely in love with and an on going battle between himself and the leader of the immortal's underground.
That would be the quicker way to summarize things, but really everything is more complicated then that. The Underground's leader has had an interest in Rowan's demon form for quite some time and while there are sudden outbreaks of human murders that are clearly linked to the underground, it would be the best time for the leader to strike against Rowan. How? Rowan honestly doesn't know. Really, all he knows is that he has a job to fulfill is the midst of this; Keep Sky safe.
Challenge now? Controlling his lust and the most inappropriate times.
Forget the crappy description, read the book.