Having sex in the company of others is called swinging. Soft swinging is having sex in the same room as another couple without contact between couples, while harder swinging may involve switching partners, for intimate activities that are defined in advance.
Closed swinging means swapping partners in private setting for more freedom and confidentiality, away from respective partners. Open swingers exchange partners in the same room, leading to more extreme experimentation, only advised for veteran swingers or people who are very secure in their relationship.
Swinging fantasies can be exhilarating and sexy – imagining an attractive couple you know in any of the abovementioned configurations can be thrilling, whether you actually go through with it or not.
But many do, and if you set and agree upon the boundaries, it can be really fun, and open new doors of possibility. There are plenty of websites talking about it – if you’re curious, check it out.