It's not really an open relationship per se. More like a swinging one. If you want this for Lena and Ian, agree to experiment together when entering a relationship with Lena, have Lena sleep with all girls including: Louise, Ivy and Emma (you can also flirt with the store clerk at the club). Ask Louise to join Lena and Ian for a threesome. After Lena sleeps with Emma in chapter 13, she will send Ian a picture of her and Emma. Have Ian respond positively to the picture and they will agree to a threesome with Emma. The next day Lena will text Ian about her trip with Ivy and will ask him if it's okay to fool around with Ivy too, respond by saying only if he gets to have a threesome with her and Ivy (this option is bugged in the current version but you can pick it with urm) then Ian will agree to give Lena a free pass to sleep with girls as long as he gets to have a threesome afterwards.