Sex Myth:
One for the guys in particular
You should jump at any chance to take part in a threesome/foursome/orgy/swinging, because it will be the best sex you'll ever have.
Multiple partners is a difficult concept to get ones head around in practice.. Particularly for men who are very prone to insecurities and jealousy in the bedroom. Movies/TV often show a girlfriend bringing another girl "to the party" so to speak as the ultimate turn on for a guy. In practice, you need to be very, very secure with your relationship to make this work - it's daunting both physically and mentally, and you are going to be on and off the pace with each other - you need to talk a lot, and concentrate on what you are doing to make sure everyone has fun. Also in these situations, a MFF threesome is often followed up at some point with the request for a MMF reciprocation. Guys who don't think this through are going to have a rough time watching their sweetheart being "fullfilled" by another guy, whilst being expected to love it. There are so many stories of guys talking their GFs into inviting their hot friend over then being mortified when they find out their 6ft 2 "best bro" with the chiselled features is next on their sweethearts "to do list".
Swinging/orgies - don't often look like they do on Pornhub! You'll get all kinds of shapes/sizes invited to the party and a strict code of conduct to adhere to! It's not usually a free-for-all, and again can be mentally draining. It's a fetish/kink, and not for everyone like the movies seem to suggest.