“Dear Daddy Kenneth: I have a reoccurring fantasy where I am sandwiched, naked, between a couple I know, having the best sex of my life. What should I do?”
Enjoy the fantasy!
If it is your intent to make that fantasy a reality, you must first decide whether the couple in question is interested in you. Have they given you any clues that they see you sexually? If not, it doesn’t mean that they are not, it could just mean that they think you are not available and respecting your existing or perceived relationship.
Let them know that you are available and interested. If they are interested and available as well, they’ll let you know. Don’t be pushy about your interest, but be genuine. They may have to talk amongst themselves before answering right away.
If they are not interested, it doesn’t mean that you are not attractive or interesting. There may be any number of reasons a couple does not return interest in you, including but not limited to: They are in a monogamous relationship, one is not interested but they are presenting a united front, they are pursing another romantic option that they don’t wish to compromise. They may not share their reason for turning you down with you, but you should respect the fact that they probably have a reason. You should also accept that whatever their reason is, it’s a good one.
Meanwhile, regardless of whether or not you get to fulfill your fantasy, there is nothing at all stopping you from having your fantasy; so enjoy it!