In most parts of the world, homophobia is in decline. The global trend is for the repeal of anti-gay laws and for greater public understanding and acceptance of sexual difference. Overall, lesbian, gay, bisexual and transgender people are gradually gaining respect and rights - not losing them.

There are, of course, frightening examples of intensified homophobic repression in parts of Africa and the Middle East. But taking the long view, in world historical terms, anti-gay attitudes and laws are on the wane.

This begs the question:

As homophobia diminishes and as future societies eventually embrace a post-homophobic culture, how will this transition to equality, dignity, understanding and acceptance affect the expression of sexuality?

If human civilisation evolves into a state of sexual enlightenment, where the differences between hetero and homo no longer matter, what would this mean for the future of same-sex desire and same-sex identity?

We already know, thanks to a host of sex surveys, that bisexuality is an fact of life and that even in narrow-minded, homophobic cultures, many people have a sexuality that is, to varying degrees, capable of both heterosexual and homosexual attraction.

It is also apparent that same-sex relations flourish, albeit often temporarily, in single-sex institutions like schools, prisons and the armed forces - which suggests that sexuality might be more flexible than many people assume.

Research by Dr Alfred Kinsey in the USA during the 1940s was the first major statistical evidence that gay and straight are not watertight, irreconcilable and mutually exclusive sexual orientations. He found that human sexuality is, in fact, a continuum of desires and behaviours, ranging from exclusive heterosexuality to exclusive homosexuality. A substantial proportion of the population shares an amalgam of same-sex and opposite-sex feelings - even if they do not act on them.

In Sexual Behaviour In The Human Male (1948), Kinsey recorded that 13% of the men he surveyed were either mostly or exclusively homosexual for at least three years between the ages of 16 and 55. Twenty-five per cent had more than incidental gay reactions or experience, amounting to clear and continuing same-sex desires. Altogether, 37% of the men Kinsey questioned had experienced sex with other males to the point of orgasm, and half had experienced mental attraction or erotic arousal towards other men (often transient and not physically expressed).

Kinsey's statistics on same-sex behaviour have since been criticised as out-of-date, exaggerated and unrepresentative. However, his idea of a spectrum of human sexuality has tended to be reinforced by subsequent surveys which have shown that a significant proportion of the population have had sexual relations with both men and women.

A British sex survey, conducted by ICM for The Observer newspaper in 2008, found that 16% of women reported sexual contact with a woman, and 10% of men said they'd had sexual contact with another man. The survey revealed a trend to greater sexual experimentation, with 23% of 16 to 24-year-olds indicating that they had a same-sex experience. All these figures are much higher than the number of people who are exclusively gay or lesbian and who define themselves as such.

The possibility that individuals can share a capacity for both hetero and homo behaviour is an idea that was researched and documented by the anthropologists Clellan Ford and Frank Beach.

In Patterns of Sexual Behaviour (1965), they noted that certain forms of homosexuality were considered normal and acceptable in 49 (nearly two-thirds) of 76 tribal societies surveyed from the 1920s to the 1950s. They also recorded that in some aboriginal cultures, such as the Keraki and Sambia peoples of Papua New Guinea, all young men entered into a same-sex relationship with an unmarried male warrior, sometimes lasting several years, as part of their rites of passage to manhood. Once completed, they ceased all homosexual contact and assumed sexual desires for women. If sexual orientation was totally biologically pre-programmed at birth, these men would have never been able to switch to homosexuality and then to heterosexuality with such apparent ease.

This led Ford and Beach to deduce that homosexuality is fundamental to the human species, and that its practice is substantially influenced by social mores and cultural expectations.

The evidence from these two research disciplines - sociology and anthropology - is that the incidence and form of heterosexuality and homosexuality is not fixed and universal, and that the two sexual orientations are not mutually exclusive. There is a good deal of fluidity and overlap.

What's more, although scientific evidence shows that human sexuality is significantly affected by biological predispositions - such as genes and hormones - other influences appear to be cultural, including social expectations, peer pressure and the availability and opportunity for sexual release. These influences channel erotic impulses in certain directions and not others. An individual's sexual orientation is thus influenced culturally, as well as biologically.

As culture changes, perhaps manifestations of sexuality can also change?

The evidence of considerable cross-over between gay and straight relations comes from research that records consciously recognised and admitted desires. At the level of unconscious feelings - where passions are often repressed, displaced, sublimated, projected and transferred - it seems probable that very few people are 100 percent straight or gay. Most are a mixture, even if they never mentally acknowledge or physically express both sides of the sexual equation.

This picture of human sexuality is much more complex, diverse and blurred than the traditional simplistic binary image of hetero and homo, so loved by straight moralists and - equally significantly - by many lesbians and gay men.

If sexual orientation has a culturally-influenced element of indeterminacy and flexibility, then the present forms of homosexuality and heterosexuality are conditional. They are unlikely to remain the same in perpetuity. As culture changes, so will expressions of sexuality.

In a future non-homophobic society, as the taboos concerning same-sex relations recede, more people are likely to have gay sex - even if only experimentally or for a few years.

Interestingly, the demise of homophobia is likely to make redundant the need to assert and affirm gayness.

Gay and lesbian identities are largely the product of homophobic prejudice and repression. They are a self-defence mechanism against homophobia. Faced with persecution for having same-sex relations, the right to have those relationships has to be defended - hence gay identity and the gay rights movement.

But if one sexuality is not privileged over another, defining oneself as gay (or straight) will cease to be necessary and have no social relevance or significance. The need to maintain sexual differences, boundaries and identities disappears with the demise of straight supremacism.

As we evolve into a sexually enlightened and accepting society, homosexuality and heterosexuality will begin to fade as separate, exclusive orientations and identities.

The vast majority of people will be open to the possibility of both opposite-sex and same-sex desires, regardless of whether they act upon them. They won't feel the need to label themselves (or others) as gay or straight because, in a future non-homophobic civilisation, no one will care who loves who. Love will transcend sexual orientation.

* For more information about Peter Tatchell's human rights and social justice campaigns: www.petertatchell.net