DORIAN SOLOT and Marshall Miller are happily unmarried. They have been together for ten years, they share a home but they have no intention of making any legal or religious statement to demonstrate their commitment to the outside world.

Instead, they are intent on resisting marriage and the discrimination still encountered by those who are not married. Marriage, they insist — and there is plenty of evidence to support them — is not a prerequisite of a successful relationship. “We met in college and were very happy living with each other for a few years, then we started feeling these gentle nudges everywhere,” says Solot. “We realised that getting married wasn’t important to us but it was important to everyone else.”

Hence their book Unmarried to Each Other: The Essential Guide to Living Together as an Unmarried Couple, and their non-profit organisation, The Alternatives to Marriage Project, that provides resources for unmarried couples.

If such proselytising seems unnecessary, it is worth pointing out that Solot, 29, and Miller, 28, live in Boston, in the US, where relationships outside marriage are often still regarded as morally dubious.

Unmarried cohabitation is illegal in Florida, Massachusetts, Michigan, Mississippi, North Carolina, North Dakota, Virginia and West Virginia, although the law is rarely enforced.

Last month a 170-year-old law prohibiting unmarried sex in Georgia was annulled, after the protests of a young chef caught in flagrante delicto with his girlfriend. The ruling decriminalised tens of thousands of cohabitees.

This, of course, is what the issue is really about: sex. It is no coincidence that in the US the most outspoken opponents of cohabitation are from fundamentalist Christian churches, says Miller: “Sex is still a scandal in the US. For a lot of people, it’s not the fact that people are doing their dishes together that bothers them, it’s the fact that they are sharing a bed. We were stunned when we went looking for an apartment and were turned away because we were not, and had no plans to get, married.”

As Solot and Miller interviewed hundreds of unmarried couples for their book, they uncovered more prejudice. “Sometimes the families of people in unmarried relationships have told them that they won’t speak to them any more. We met one unmarried couple who were banned from their friends’ house because the married couple didn’t want their children ‘exposed to that lifestyle’,” says Solot.

The US Census Bureau specifies that there are 9.7 million cohabiting Americans, or less than 4 per cent of the population. Between 1990 and 2000 the number of unmarried couples living together increased by 72 per cent. In England and Wales cohabiting couples account for 8.3 per cent of households, and the proportion of married households is down to 45 per cent, according to the 2001 census.

The proportion of unmarried couples cohabiting before marriage more than doubled between 1979 and 1999, which has created a greater tolerance. But tolerance does not necessarily stop parents and grandparents from encouraging matrimony. Kate Cohen, while attending her sister’s wedding, was asked 32 times when she and her partner planned to marry.

Many cohabitees can also recall situations where not being married raises a bureaucratic roadblock: for example, company benefits not extending to a partner, or being charged more for a second driver by a car-hire company.

So if being unmarried is this tricky, why are increasing numbers of couples avoiding the ceremony? The reasons are plenty and not, as many of the unmarrieds’ critics would suppose, because of a disinclination to commit.

“One of the reasons for not getting married is that I don’t feel the state has a right to say who you can sleep with, who you can have a kid with, who you can live with, and who you can split up from,” says Becky, who has been in an unmarried relationship for 21 years. “Of course, when I said that, my parents thought the main reason I was doing this was so I could split up whenever I felt like it.”