It has happened to varying degrees among all of the couples in our group, but sex is only one of the drivers. The woman who I am most attached to, other than my wife, is not the one who I have sex with the most, although we do frequently get together; she is the one wth whom I share common interests that our spouses don't. It is the woman who I have most often gone on overnights, long weekends, even a vacation by ourselves. Even that, however, has made me appreciate my wife more. The relationship with the woman who I am attached to is like two dimensional - our shared common interests and sex. With my wife it has many more dimensions besides sex and common interests: family, house, our future dreams.