Oh dear. So you've met a sexy couple, had raunchy good times with them, then gradually started seeing them on a more social level and BAM! Suddenly you're not really interested in having sex with them anymore? Sounds like you're in a relationship!
This isn't too far from the truth however. If you spend enough quality time with someone, you end up being friends and sex is probably one thing that isn't a huge priority between friends, if at all! Given that, it's not unusual to not feel sexually charged around people you've cooked snags on the BBQ with or whose children you've helped clean up after a birthday party.
Love and affection keep a couple hot and heavy for each other but since it's inappropriate to have those feelings for another couple, there's nothing there to keep the embers burning for a sustained sexual relationship. Naturally it will be more exciting to meet a new couple in the end.
It's worth considering if your friends may be feeling the same way. If they are then you'll know because your relationship will be based more and more on things that make other 'regular' friendships happen. However, if your friends are still keen to try and get you naked, then you'll either have to be honest with them and possibly lose a friendship (they may be keen to have a more longer term sexual relationship with people they consider friends), or subtly steer things into neutral territory until they get the hint. You can do this by limiting your dates to social events only. Ultimately, if they are not into that and don't want to maintain a regular friendship with you, they will move on. It will all be for the best.
And how to keep this from happening again? If you're a type of couple who make and keep friends easily, this may be hard but you'll need to consider limiting your social contact with swinging couples and simply meet them for down and dirty sex. You may find that this is the reason why some couples prefer to not form friendships, as cold as it may seem.