Sexualizing a situation is actually the last thing from me or my boyfriends mind when we are on MDMA. I am extremely friendly and affectionate, and it's often met by people I don't know well hoping it's flirting/swinging. We encourage friendly interaction with others (light shows, hugs etc.) but taking it to the next level seems completely foreign while on MDMA.
I think if you enjoy sharing your partner in sexual scenarios while rolling, it's part of something you get turned on by normally, but MDMA may allows you to feel more comfortable with the idea, or that aspect of your sexuality.