Married people who no longer have sex obviously differ from the virgins and singles in that they did have sex at one point.
"To some extent this makes it harder for them to speak up," Burgess says. "They are still very much in love with their spouses. They say, 'Well, I am not sexually satisfied but he is a good father and provider and a fun person -- I just wish he would find me sexually interesting.'"
Sex therapist S. Michael Plaut, PhD, is associate professor of psychiatry at the University of Maryland School of Medicine and immediate past president of the Society for Sex Therapy and Research. Asked by WebMD to comment on the Donnelly and Burgess findings, he notes that their Internet-based study may actually have underestimated the problem by missing older people who don't use computers. And he would add a fourth category to the list of people who want but don't have sex: married people who later find that they are gay or lesbian.
Both Plaut and Burgess recommend that people who are troubled by their lack of a sex life should seek professional help.
"The message for anyone experiencing a period of involuntary celibacy is they are not alone," Burgess says. "One thing that has been very helpful to some people is to find community. This can be done on the Internet."