My current sexual fantasy is to be in a polyamorous relationship. Specifically, a 'polycule triad'. This would be a very complex relationship where people can be primarily or secondarily linked to others involved in the relationship. For example, John and Jessica are married, but Jessica has a girlfriend on the side that John knows about, and the three mutually support the relationship. Such a polyamorous relationship would be ideal for me.
I envision a very unique partnership lifestyle supported with two other people that functions better than a traditional monogamous relationship.