This friend sharing, dubbed "partner betweenness," happens when a man's girlfriend or wife becomes better friends with his buddies than he is. One in four men in the study experienced this phenomenon. These men were 92% more likely than men who did not experience "partner betweenness" to struggle with ED.
Researchers believe the resulting difficulties with ED are not caused by jealousy or suspicions of cheating. Rather, men need male friends to boost their sense of masculinity. Think about it this way: if a man is out with a male friend who is close with his wife, he will not feel as free to vent about relationship problems or even admire other women for fear that this friend will betray his confidence. If a man's female companion steals one of these male friends, she is in a sense stealing elements of his masculinity.
The study examined data from a 2005 survey of over 3,000 men between 57 and 85. Even after researchers controlled for common health problems associated with ED, they still found a strong link with partner betweenness and sexual dysfunction. For men between 57 and 64, partner betweenness had just as strong of an association with ED as prostrate issues. The problem even appeared among the healthiest men in the group.
The association with ED and partner betweenness fades as a man enters his 70s and 80s. Researchers believe that this is because a man's understanding of his masculinity changes into more of a grandfatherly role.
The researchers make it clear that sharing friends can in fact foster a sense of "couplehood." However, a woman must be careful not to adopt her partner's buddies as her personal friends and to allow her boyfriend or husband space to build his sense of masculinity and camaraderie separately with his friends.
Read more about this study at TIME.com.