I think society has evolved to a point where this type of behavior is acceptable, while there may be some in religious organizations that will call this behavior a sin I believe overall there is nothing wrong with it, as long as these people are enjoying themselves and having fun I don't see it as wrong and it's completely natural.
I feel that bisexual promiscuity of swinging singles and swingers is more natural than it is a sin. It might not be natural for everyone but to be considered a sin is wrong. For thousands of years humans have been known to have multiple partners in relationships, and to this day when people do it they are acting in no different way.
I see no reason for the sexual practices of consenting adults to be labeled. I believe as long as there are no children involved and no one is harmed, whatever consenting adults do among themselves, in private, is not a sin. Sex is natural, but not everyone is cut from the same cookie cutter.
Bisexual promiscuity is a natural instinct, one only need look at their sexual preferences and find they originated when they sexually matured. As we can see that sexual predilictions can vary from child to child within the same household, it is clear that the preferences are, for the most part, naturally wired.
I think some people are more apt to be promiscuous and I think this can change throughout a persons life, so to say it is a sin is more about the accusers personal taste, especially in regards to their religion. I personally believe it is bad to judge people based on your religious beliefs because most religions tell a follower that judging is not their job.