You will know when Morrigan wants you to have sex with her because in camp she will tell you that it is cold in her tent and that you should come in with her. This will start up her specific sidequest which you can complete for approval points (I didn't complete the sidequest and I was able to have sex with her).

Bring Morrigan in your party to the Mage's tower and she will tell you of a grimoire she wants you to find. Try to get her to talk about a mirror that Flemeth took from her and then go to a specific merchant in Orzammar that sells a mirror as a gift. She likes necklaces and amulets as gifts, one easy one is sold by the merchant in camp called Golden Rope Necklace. Morrigan also dislikes when you make decisions to help people when they don't directly help you end the Blight she hates being sidetracked. Make sure to never suggest to her anything about love being necessary or good. Morrigan is tricky because she doesn't want to be in love. NOTE: Leliana is bisexual and you can score with her as a boy or a girl character.

Give it to her for a large approval bonus. Go to the Brecilian Forest and near the entrance is the flower Andraste's Grace. She prefers holy symbols and items of Andraste as gifts as well as shoes. You can also buy her a nug in Orzammar to increase her approval. This will start a side quest that, when completed, will give you massive points with Leliana. By going through her dialogue options, eventually one will open up asking something like 'I have heard that in Orlais traveling minstrels are often spies'. One good way to get her approval up is to do her optional side quest. I found Leliana to be the most difficult character to score with. NOTE: Alistair is straight and to score with him you must play as a girl character. You need to have him around 90% approval to get in the tent with him. One gift to get him easily is his mother's amulet that you can retrieve from Castle Redcliffe in the Arl's study. To actually have sex with the character, they will either confront you in camp asking you to go to their tent or you ask yourself.ДЄlistair's conversation options open themselves up pretty well and it is easy to predict what things to say that will make him love you more. For this achievement, you have to have sex with these 4 characters (can be in as many playthroughs as you need).

Dragon age origins morrigan romance