Single swingers and swinging means someone is bisexually promiscuous because the very nature of swinging is to have multiple partners. When someone is in a monogamous relationship, they do not swing with other partners and therefore are not promiscuous because they are part of a smaller group.
Once you start to swing with other couples, whether they are heterosexual or homosexual, the group has become larger and therefore promiscuous. When a single swinger is sexual with a heterosexual couple, there is the likelihood of bisexuality.
Just because someone is in the swinging lifestyle does not mean that they are bisexual. They might just be in that lifestyle for their partners enjoyment and they could just be tolerating it. Or they might just want to have sex with the opposite sex. Not all swingers are bisexual.
Nobody will deny that some people that are both bisexual and promiscuous are also involved the swinger scene. There is nothing wrong with those people, however, it is false to assume that everyone within the swinger community identifies as bisexual or promiscuous. Some swingers may only 'swap' with the opposite gender of couple groups. Some swingers may not actually go out and engage in these activities often. How can we classify an entire group by such strict terms?
While a lot of swingers are bisexual, it does not mean that they all are. In fact, most are not and are just looking for another person to share their sexual experiences with. It should not be stereotyped as anything besides that. Sex between multiple consenting adults. That doesn't mean bisexually promiscuous.
Just because someone chooses the swinger lifestyle it doesn't mean that they have elected to have bisexual relations. In fact the majority of people classify themselves as swingers also only engage un heterosexual relationships. Swingers choose to have a variety of partners, but it doesn't mean they are choosing to have partners of the same sex!