When a music teacher discovers his new girlfriend is a serial killer, he must decide whether to turn her in or continue having amazing sex.

Dilemma does not necessarily equal action.

No action no story, what is the action in this story? What will the MC actually do?

A dilemma, ipso facto, entails conflict. The best conflicts create dilemmas.

As love stories go the best have a force of opposition to the love trying to keep them apart. Other wise the "horny" dilemma aside it is a simple case of they love each other and that is it without conflict it becomes a boring account of two people getting together.

Once he decides what he wants to do i.e stay with the girl he needs to fight the force of opposition to the love in this case the law comes in handy. He could fight the police to protect their relationship as she keeps killing.

But all this needs to be clear from the logline not just the nature of his initial decision and their relationship.

This has the making of a good black comedy but I strongly recommend adding in a point of conflict not just dilemma.