The thought of involving an additional person in your lovemaking is a big turn on for many couples some may prefer a second male while others like an extra woman.
If you involve and additional partner either a male or female the arousal comes from the same desires.
Both options give one a chance to see their partner with someone else while the other enjoys seeing it happen.
The desire may be bisexual fantasies usually between the women but sometimes the men.
Foursomes moresomes and swinging:
Couples may have foursomes if they prefer to just swap partners and thus may then be termed swingers however it may not always involve an extra male and female couple but sometimes several men or even women could be involved in a group.
Swinging:
Gone are the days when you heard of couples throwing their car keys in a hat and the wife having to go off with the owner of the keys she picked in a drunken party, now the women are in control and say who they want to join them.
Couples may carefully select potential swinging partners through contact adverts or at clubs and parties, they will try to find similar partners having same tastes and desires or even common fetishes.
Oral sex:
Believe it or not oral sex is just something of a fantasy for some partners. Some consider it dirty and would not consider doing such an act, many would however enjoy there partner to practice it on them but be put off by kissing after.
Watching your partner enjoy the experience of you performing oral on them is very pleasing and often produces as much pleasure as actually receiving oral. Both to perform oral together in the famous 69 position is extremely pleasing as you both entwine closely.