Marty Ralph Klein (born 1950) is an American sex therapist, author, educator and public policy analyst. Klein has spent his career supporting the healthy sexual expression of men, women and couples in a wide range of ways. He is critical of censorship, the concepts of sex addiction and porn addiction, as well as the anti-pornography movement. He believes that public policy relating to sexuality should be driven by scientific data rather than emotion, "tradition" or popular but untrue myths. He has been a participant in various state, federal and international court cases dealing with the First Amendment, obscenity, censorship and "harmful to minors" laws.

Klein grew up in Brooklyn, New York. While still in elementary school he began to play the recorder and to collect stamps, interests he has continued throughout his life. Collecting stamps led to a lifelong interest in geography and history. He later wrote on these topics frequently when he began to lecture and travel internationally.

After graduating from Stuyvesant High School in 1967 he attended Stony Brook University. There he developed a passion for sociology and went on to attend PhD programs in sociology at Indiana University and the University of California.

Klein was trained in the doctoral programs of two different branches of sociology: the first was survey research, the use of statistical analysis to gather data about human behavior in order to understand, correlate and predict it; the second was ethnomethodology, which is the study of how people create meaning as a prelude to creating orderly social interactions and predictable social institutions.

First as a volunteer and then as a staff member, Klein worked for the Santa Barbara branch of Planned Parenthood (1976–1980). While there he became intrigued with the recurring experience of women returning for pregnancy tests multiple times despite being prescribed or given various types of contraception. These women's explanations surprised him: they didn't want to use birth control because they were afraid their partner would think they were a slut, or that they had actually planned to have sex with a stranger they'd just met at a bar.

Planned Parenthood then asked him to run a group for the male partners of women coming to the birth control clinic. He also received a grant from the state Office of Family Planning relating to male sexuality. His interest already piqued by his experiences at the clinic, he began his career in human sexuality.

Klein has been outspoken about the way sexuality is discussed in media outlets. For example, a 2005 New York Times article on the phenomenon of self-help books about sexual positions, sex fantasies and increasingly edgy materials stated that the genre is big business, aimed at women and promoting the idea that "It is a woman's role to ensure that the couple's sex life remains satisfying." Klein disagrees that the promises that these books make about improving sex with oral, anal and fetishistic techniques and information are not what most couples really need to make them happy. "A book called 'How to Get Your Wife to Hug You a Little Bit More' or 'How to Get Your Husband to Slow Down and Caress Your Hair and Love Doing It,' now those are books that would change people's lives," says Klein. Communication is the key to satisfying relationships; things like new positions or removing pornography from a home without your partner's consent is generally not helpful. Klein told the Commonwealth Club that what most adults want out of sex is a combination of "pleasure and closeness," and he encourages people to pay more attention to these, rather than to performance anxiety or how they look.

Klein has criticized the mass media for talking about sexuality in what he claims is an exploitative manner. He calls this the "Oprah-ization" factor, where talk shows like Oprah and Dr. Phil will, for example, put teen prostitutes on stage and talk about how awful it is. What they are really doing, according to Klein, is showing teen girls in skimpy clothing talking about sex, which results in voyeuristic viewers. "If the American media really thought these stories were so terrible it wouldn't give them so much air time ... The key message in American culture is that sex is dangerous. But sex isn't dangerous, bad sexual decision making is dangerous." In an interview with Chip August for Personal Life Media Klein stated, "I think Oprah has single-handedly launched the victim industry in this country," adding that society is now infantilizing women by saying that they are unable to make decisions for themselves, that they are tricked into drinking at parties, that they can't control whether they get drunk or create circumstances of vulnerability. "It's demeaning to people to say that even though you're an adult, we're not going to hold you accountable for your own decision-making," he says.