Soft sex is when you have sex on your own, or with a partner, in the company of one or more others, with physical contact occurring only with the consent of everyone in the room. In this context, the definition of sex is very broad and includes kissing, cuddling, erotic touch, watching porn together, watching others have sex, masturbation, manual sex, oral sex, bukkake and sexual intercourse. Soft swinging is an example of soft sex.
Sarah: Wanna come home for soft sex with me and Dan?
Kate: Yeah, I'll watch you both and maybe kiss you if I'm in the mood, but I'm not up for fucking Dan.