The public health needs of bisexual men are often left unmet, leaving them vulnerable to a number of health problems, including sexually transmitted infections (STIs), a new study has found.

According to researchers at the US Centers for Disease Control and Prevention (CDC), while men who have sex with both men and women represent only a small portion of the population, they are disproportionately affected by STIs, including HIV.

The researchers found that there may be a number of reasons for this, including having a larger number of sexual partners, forced sexual encounters, having sex from a younger age and substance misuse.

Social factors may also play a role, such as negative attitudes towards bisexuality and ideas about masculinity.

"Biphobia can manifest in erroneous beliefs that bisexual men are gay men who have not disclosed their sexual orientation and are responsible for HIV transmission to women. Experiencing these sentiments can contribute to bisexual men's social isolation and psychological distress, which in turn may promote HIV/STI risk through substance use, sexual risk behaviors, and the avoidance of prevention services," commented the study's author, Dr William Jeffries.

The study noted that HIV prevalence is lower among bisexual men than homosexual men. However, it is higher than the rate among heterosexual men. Furthermore, bisexual men are less likely than gay men to get tested for HIV, which can lead to undiagnosed HIV.

Meanwhile, the researchers found that attitudes towards pregnancy can also play a role.

"Bisexual men may avoid condom use when their female partners use other contraceptives or when female partners perceive condom use as a sign of relationship infidelity.

"Further, bisexual men's desires to produce offspring biologically may prompt sex without a condom with female partners. In this regard, desires for fatherhood may indirectly increase these men's vulnerability to HIV/STIs and transmission of these infections within their sexual networks," Dr Jeffries said.

Details of these findings are published in the American Journal of Preventive Medicine.