This book contains reasons why men who have had sex with both men and women choose not to disclose their sexual orientation, particularly to their wives and girlfriends. It also aims to present safer-sex education, instructing individuals on how to reduce the risk of contracting disease and encouraging them to use caution with each partner.
You can define the "down low" any way you like. "Living on the down low" refers to the behavior of men who sometimes have wives and very loving, intimate relationships with women, but have sex with other men in secret. Yes, their wives or girlfriends have no idea of the double lives they are leading. This also refers to women who are in relationships with men and are having secret sexual relationships with other women.
These people can be members of the clergy, bishops, pastors, doctors, construction workers, businessmen, truck drivers, college students, young, old, black, white, any culture, any nationality. You cannot look at them and tell. They can be the most masculine, macho men around. They do not consider themselves homosexual, and may not like to be around openly gay men in public. They may even act and sound homophobic.