Sex: Buddies in the Bedroom

Friends play an intimate role in sex, even for the most private couples. Partners who share more mutual friends have longer, more satisfying relationships, research shows, and overlapping social circles are associated with decreased likelihood of cheating.

But friends aren't always a boon to the bond. According to a new study, they can lead to performance problems. A middle-aged man whose partner talks with his confidants more than he does is twice as likely to suffer from erectile dysfunction (ED) as a man whose partner keeps her distance from his buddies. Researchers suspect that when a woman "hijacks" a man's friends, his sense of autonomy and privacy—central to his masculinity—plummets.

Often, 55- to 65-year-old men are retiring from their jobs. "They're segueing from expectations to be assertive and in charge in the workplace to an apparent loss of responsibilities," says University of Chicago researcher Edward Laumann. "Sorting out these new circumstances is stressful."

When a man already struggling with his self-image feels his partner is closer to his buds than he is, the stress may result in physiological difficulties in bed. Indeed, other research has linked stress and ED. And the effects, Laumann says, "are comparable to those of prostate inflammation on ED."

Luckily, those effects wane. For men aged 65 to 85, having a chummy partner doesn't correlate with ED. Older men may have let go of the need to fit typical expectations of masculinity, Laumann says, so they're less likely to feel threatened by a partner's closeness to mutual friends.

Younger men can take a cue from elders and see their significant other's friendliness as common (it happens in about one in four couples) and not threatening. "It does not necessitate a loss of control," Laumann says. "Men should keep this in mind to relieve self-stress and even enjoy their newfound freedom."

Friend or Foe?

Your social circle affects your sex life at every age, according to other research by Laumann. One study of 18- to 59-year-old men divided subjects into two groups: those who had had 13 or more sexual partners and those with 12 or fewer. In the 13-plus group, people with more friends reported fewer STDs. But in the 12-or-fewer group, more pals meant higher STD risk.

The more promiscuous men were often single and "hooking up with people at gatherings," where their buddies could warn them against risky pairings, Laumann says. But promiscuous guys with few friends lose out on such coaching and are more likely to wind up with STDs.

Most of the men in the 12-or-fewer group were in relationships. Those with many pals were likelier to contract STDs because their partner couldn't keep an eye on them—and they were cheating with strangers, without background checks from friends. In contrast, guys with few friends were likely to be STD-free because their partner's monitoring kept them faithful.