We don't know. In spite of people's curiosity about the topic, there has never been an experimentally controlled study conducted to answer the question.
Why not? Just imagine the study: It would likely require asking study participants to follow a prescribed diet while their partners – who would need to be blind to their significant other's diet (tricky if they spend much time together or live together) - performed oral sex with some regularity. In doing so, the partners would also have to taste the men's semen or women's vaginal fluids and then record their ratings.
If any of the "tasting partners" were women (girlfriends, wives, etc.) then the research would also be wise to take into account women's menstrual cycle as women tend to be more sensitive, and thus possibly have their sense of taste affected, during certain parts of their menstrual cycle if they are not on hormonal birth control.
It would also be important to track the menstrual cycle of women being tasted given that their own vaginal scent and taste can be influenced by their menstrual phase.
So, for one: this isn't an easy peasy study to do (though it's certainly doable). Second, funders aren't exactly beating down our door to perform this kind of research – but, if they were, I'd totally do the study. The benefit to people? They'd finally know whether spending their money on pineapple, kiwi, and other fruity juices is worth it. The benefit to me? I'd finally have an answer to a question I'm asked almost every single week.
All that said, some medications affect the taste of semen and vaginal fluids. For example, some people taking oral antifungal medications report a metallic taste to their fluids. And of course people's taste buds - and taste preferences - vary. One person's yummy fluids is likely to be another person's yucky fluids. Given that odor often affects taste, basic hygiene (showering regularly and after workouts; wearing clean underwear; prepping before you're likely to receive oral sex) may go a long way toward creating a more pleasant oral sex experience for your partner - unless she or he is into the dirty smell (which some people are), in which case have at it.
This December, famous sexologist and Gizmodo friend Dr. Debby Herbenick-author of Sex Made Easy and Because It Feels Good: A Woman's Guide to Sexual Pleasure and Satisfaction-will answer your sex questions. Every day, Dr. Herbenick will pick a question (click here to see them all and add more) and give you a solid, scientific answer.
Dr. Debby Herbenick, author of Sex Made Easy and Because It Feels Good: A Woman's Guide to Sexual Pleasure and Satisfaction, is the Co-Director of the Center for Sexual Health Promotion in the School of Public Health-Bloomington at Indiana University (IU) where she is a Research Scientist. She is also a sexual health educator at The Kinsey Institute for Research in Sex, Gender and Reproduction where she writes (and hosts audio podcasts of) the Kinsey Confidential column and coordinates educational programming. She has a PhD in Health Behavior from IU, a Master's degree in Public Health Education (also from IU) and a bachelor's degree in psychology from the University of Maryland, College Park. In addition, she is certified as a Sexuality Educator from the American Association of Sex Educators, Counselors and Therapists.