When one partner wants sex more than the other, masturbation is an acceptable and understandable alternative. Some women are upset, even outraged, if they find their partners masturbating. Masterbation is probably universal for females as well as males. It generally represents an indivudual's introduction to sexual pleasure and self love. In this day of AIDS, increased incidence of sexually transmitted diseases, solo sex is a safe outlet for the healthy sexual impulses.
Couples devote much time, energy and resources raising their children.
This leaves less opportunity to attend to individual and mutual needs. They don't have time to do things for themselves and for each other. When the children leave home, husband and wife now have more time to renew issues put on the back burner. They can restore their sexuality which probably has taken a back seat to raising the kids. Now, they don't have to wait for the children to fall asleep or be away at camp.