Women’s definitions of lesbianism appeared to permit more flexibility in behavior than their definitions of heterosexuality. For example, of the women who identified as lesbian in the last round of interviews, 15 percent reported having sexual contact with a man during the prior two years. In contrast, none of the women who settled on a heterosexual label at that point reported having sexual contact with a woman within the previous two years.
“This provides further support for the notion that female sexuality is relatively fluid and that the distinction between lesbian and bisexual women is not a rigid one."
Full text of the article is available from the APA Public Affairs Office or at http://www.apa.org/journals/releases/dev4415.pdf I plan to take a look at the full study before commenting further.