Reinisch, who is now director of Acquisitions and New Exhibitions at the Museum of Sex in New York City, pointed out that the older men in the study were unlikely to label penile-vaginal intercourse as "sex" if they were using a condom. One hundred percent of the men in the 18 to 29 age group called penile-vaginal intercourse with a condom sex, but only 82 percent of the men in the 65 and up age group considered sex with a condom sex.
Reinisch said this "reflects how a significant portion of individuals born during a certain era feel about or understand a particular phenomenon."
People who came of age before the contraceptive pill was widely available may then define intercourse as sex only if there's a possibility to procreate.
"Perhaps for older men what really counts ... is the act that is involved with procreation. The act that was primarily thought of and discussed as 'the sex act,'" Reinisch said.
Ed Laumann, a professor of sociology at the University of Chicago, said these differences require social science researchers to be really careful about asking questions in studies.
"It's been a discussed problem for a long time," said Laumann, who also doesn't believe we will ever come to a consensus as a society about what it means to have sex.
"Well, what is the answer? There is no true objective thing here," he said.
While changing definitions might be interesting for sociologists, the shifting definitions could cause doctors problems.
Coleman, of the University of Minneapolis' Program in Human Sexuality, said, "What kind of sexual activities a person is having will determine what kinds tests and where to test them."