In the largest study ever on obesity and sexual health, it was found clinically obese women were much less likely to seek advice on contraception or take the pill.
Researchers found obese women were 29 per cent less likely to have had a sexual partner in the previous 12 months.
The were also five times more likely to have met their partner on the internet, and were more likely to have an obese partner than women of a healthy weight.
However, obesity also has a huge impact on men, with obese men being 69 per cent less likely to report more than one partner in the previous year compared with men of normal weight.
The study found they were almost three times more likely to report erectile dysfunction as men of a normal weight and were more likely to say they had picked up a sexually-transmitted infection.
The research, published online in the British Medical Journal (BMJ), involved more than 12,000 men and women in France.
But a British expert said the findings had far-reaching implications, and called for greater understanding of how obese people feel about their sex lives and the impact of obesity on health.
In an accompanying BMJ editorial, Dr Sandy Goldbeck-Wood, from the Sexual Problems Clinic at the Camden and Islington Mental Health Trust in north London, said: ''In particular, we need to know why obese women use less contraception and have more unwanted pregnancies despite having fewer sexual partners.
''The answers are likely to be complex, with biological, psychological, and social aspects.''
The French study was carried out on people aged 18 to 69.
When it came to seeking advice on contraception, taking the Pill and unplanned pregnancies, women under 30 were most affected.
The authors, from the French Centre for Research in Epidemiology and Population Health, wrote: ''Prevention of unintended pregnancies among these women is a major reproductive health challenge.
''Healthcare professionals need to be aware of sensitivities related to weight and gender in the provision of sexual health services.''
Dr Goldbeck-Wood said the research should help doctors address topics they find hard to discuss with patients, such as sex and obesity.
''In public health terms, the study lends a new slant to a familiar message: that obesity can harm not only health and longevity, but your sex life.
''And culturally, it reminds us as clinicians and researchers to look at the subjects we find difficult.''
