Andrew Smiler, author of Challenging Casanova: Beyond the stereotype of promiscuous young male sexuality, says that men don't realise they can actually refuse sex.
He said that guys find it hard to say "no" because they're concerned about how the girl will respond, including concerns that she'll think he's either some type of wimp or gay.
Dr Smiler, former president of the Society for the Psychological Study of Men and Masculinity, recently wrote A Guy's Guide on When to Refuse Sex for the Good Men Project about when men should refuse sex with a new partner.
He said guys are often uninterested in sex if they're tired, sick or too drunk to perform sexually.
A recent survey for online pharmacy ukmedix.com found 62 percent of men turn down sex more frequently than their female partner, with a third admitting they had lost their sex drive.
Another poll reveals one in four men no longer has sexual intercourse at all - and the figure rises to 42 per cent for men over 55 - while a quarter said they had been affected by erectile dysfunction at some point in their lives.