The majority of swinger couples have probably been together for a long time and generally understand each others needs, desires, and history. Some people find is surprising, that HIV hasn't exploded in the swinger community like it has in the gay community.
Obviously there are play couples who are not careful about safer sex or building relationships prior to sex. At swinger events people often swap partners without giving a lot of thought to who they will be having sex with. Also common are small orgies, where safe sex is frequently optional, and protected oral sex can be non-existent. But since 1986 there has been only one reported case of HIV infection in the swinger community.
For a start, the lack of intravenous drugs in the lifestyle. Intravenous drugs are well known to be a primary way in spreading HIV, this is because they are generally looked down on by the swinger community. This most likely stems from the social standing of the people who are in the lifestyle, normally they are married, hold a professional job, perhaps have children; they are every thing that does not define a typical hard drug user, one which would share a needle.
Another reason is the amount of bisexual males, due to social reasons; bisexual males are not as welcomed in the lifestyle as bi sexual females are. It is possible if female swingers expressed an interest in for male-make contact, that the amount of male bisexuality would increase, but at the moment, they make up a low percentage. Statistically, bisexual and gay males have carried an extremely high risk of HIV, so this would account for the avoidance of the disease in the lifestyle.
Honest sexual history: when couples use swinging in stable relationships it can prevent lying, people that fear the HIV and AIDS situation have been known to downplay there sexual hasty and risk groups. Because swinging is an open style relationship that doesn't require the need to lie, a persons full sexual history can be known, thus reduces the risk of disease.
Not all couples in the lifestyle have non discretional sex; the majority of couples prefer social and sexual relationships with other couples, they see swinging as a social behaviour, sex is just part of it.
Couples may insist on getting to know people socially before going onto sexual activity, have regular couples they see, or maintain the lifestyle with people who's STD status and is common knowledge. Not all couples have unprotected sex, according to a recent study, 62% of swingers began practicing safer sex after it came clear that HIV was spreading through un-protected sex.
10% of the swinging population attend functions solely to socialise, other may prefer to "soft swap", this is changing partners for heavy petting, they return to the partners for actual intercourse. Some lifestyle couples have a fluid-monogamous relationship, which is neither of the couple share to share body fluids with any one else apart from each other.
Which ever combination of the reasons above, it is clear that the swinging community is doing something right, and hopefully this will carry on for years to come.