This page contains all the information you need on how to complete the romance with Morinth.

After you’ve completed the necessary steps to beginning and building the romance with Morinth which can be found here, How To Start Morinth Romance. Once you’ve returned from the loyalty mission, speak with Morinth and say “How are you?” followed by “Yes” to engage in an intimate moment with her. However, before you decide to go ahead with the scene, make sure to create a quick save, as once the scene has come to a close, Shepard will die.