The Marriage (Same Sex Couples) Bill is continuing its rocky passage through the House of Lords after surviving a “wrecking amendment” last month. But opponents have not given up.

There are more debates, amendments and votes in the Lords next week, and one attention-grabbing campaign group has contacted every peer with a view to derailing the legislation, which will allow religious organisations to “opt in” to offering weddings to gay couples.

Gay Marriage No Thanks (GMNT) is an offshoot of Anglican Mainstream, the Christian coalition group that wanted to put full-length adverts on the sides of London buses suggesting that people could be cured of homosexuality.

GMNT has taken out ads in the Times offering “10 good reasons” why the Lords should scupper the bill. Reason number one is: “Intact biological families provide the gold standard for the wellbeing of children.”

In a press release sent out this week, the group expands on this theme with a slogan that reads: “FACT: children do best with married birth parents.”

In a separate line of attack, the group also targets sex education in schools, saying that a change in the definition of marriage will automatically lead to children being taught “the biological mechanics of both gay sex and procreative sex”.

Spokesman Alan Craig puts it more bluntly, saying: “Parents will be shocked to learn that little Sophie or Billy will have to be taught formally about anal sex by the school authorities. As a father I object strongly. It is medically unhealthy and utterly inappropriate.”

Assuming this kind of language hasn’t got us blocked, let’s look at GMNT’s claims one by one.

“Children flourish best with the commitment and stability of their married birth parents…this traditional domestic arrangement is significantly better for children than all others including same-sex parent households.”

GMNT provides links to a number of research papers and studies to back up this idea. But most of them are irrelevant to the question of whether gay parents are worse than straight ones.

That leaves us with two papers worth looking at. One, by US sociologist Loren Marks, reviews 59 studies published by the American Psychological Association on lesbian and gay parenting.

None of them found that children brought up by same-sex couples experienced any significant disadvantages in later life.

But Mr Marks found almost all the studies wanting. He said most of them were based on small, unrepresentative groups of people. Most of the gay couples tended to be white, well-educated and middle-class, for example, which would tend to skew the results.

He concluded that the US psychology establishment has been too hasty in saying that all the available scientific evidence backs same-sex parenting.

That’s not the same as saying that gay parents don’t do as good a job as straight ones. Mr Loren simply says that evidence isn’t yet good enough to call it one way or the other.

The main source of ammunition for GMNT on gay parenting is this massively controversial study by University of Texas sociologist Mark Regnerus.

The paper caused a storm in America by apparently turning all the previous evidence on its head.

Mr Regnerus found that the children of gay parents scored significantly worse in later life than people brought up by both birth parents on a range of indicators including educational attainment, employment, income, crime, drug use and mental health.

There was a basic problem though. The “gay parent” group was not made up of people brought up in the kind of stable family environment that supporters of same-sex marriage have in mind.

People were placed in that category if they believed that one of their parents had “ever had a romantic relationship with someone of the same sex” at any point during childhood.

Of the 248 people who said one of their parents had some kind of gay relationship, only two lived with their mother and her partner from birth until the age of 18.

Arguably, this made the study more of an exercise in comparing volatile parental relationships with stable ones, rather than a gay vs straight thing.

The publishers of the study asked another sociology professor, Darren Sherkat, to look at the paper, which was part-funded by the conservative, anti-same sex marriage Witherspoon Institute.

Professor Sherkat said the research was “bulls***” and should never have been published.

Mr Regnerus has defended his work and his university concluded that there was no evidence of scientific misconduct.

“If the Bill becomes law, children as young as 12… will have to be taught the biological mechanics of both gay sex and procreative sex in the context of the national curriculum requirements to promote and explain marriage”.

We’re a bit baffled by this claim, which seems to be based on linking up different bits of the law on sex education which don’t quite fit together.

The Education Act says pupils must “learn the nature of marriage and its importance for family life and the bringing up of children”.

It’s plausible that if the legal definition of marriage is changed to include same-sex couples, that will mean that schools have a legal obligation to teach children about gay marriage.

That doesn’t mean there is any obligation to go into detail about the mechanics of gay sex in the classroom.

In fact, the Education Act also says children should be “protected from teaching and materials which are inappropriate having regard to the age and the religious and cultural background of the pupils”.

And if it’s all too much for Billy and Sophie, parents have the right to pull their children out of sex education classes, which the exception of the core subjects covered by the national curriculum for science.

These compulsory bits touch on sexual health but are mostly about “the human reproductive cycle including adolescence, fertilisation and foetal development”.

In other words, this is “where babies come from”. By definition, homosexual sex is not included.

Having spoken to GMNT, their real concern appears to be some kind of challenge under the Equality Act if a school is teaching the mechanics of straight sex but not giving gay pupils the equivalent information.

This may or may not be a live possibility, but it’s difficult to see what is has to do with gay marriage.

By Patrick Worrall