You decide that the fact that "you" answered the door is a sign and that you should instead reclaim your old life by merging with it right now. You set "you" as dominant since you want to resume your old life as a hot babe. No way are you giving the pleasures this body has made you feel, to hell if it erases your relationship with Lucy.
You are now a hot babe coming back from a day out shopping, as you go through your new memories to assess the changes you realize that
A. Lucy is still your girlfriend but now you are a bi couple as you two have a man that you like, seems the device changed her orientation to keep things consistent.
B. Just Lucy's family was also changed when she merged with Amelia, yours changed when you merged with this babe.
C. The device always picks the most desirable outcome; You wanted Amelia as a GF, she became your GF Lucy, you didn't want to lose that when you turned yourself into a babe, now she's bi.