So Penny Wong, our finance minister, is going to be a mum. Sophie Allouache, Wong’s long-term partner, announced on Tuesday that her IVF-conceived baby will arrive in December.

While many commentators expressed appropriately genuine joy for the expectant couple, the news has also flushed out a predictable flow of concerned opposition from socially conservative quarters.

Predictably, Christian Democrat politician Fred Nile thinks it’s a bad idea. The list of things Fred Nile opposes is pretty long, but it features just about anything to do with homosexuality and children being raised by same-sex parents.

Not only does Nile oppose outright the idea of two mums raising a baby, but if they must do it he’d rather they kept their family life in the closet.

“She needn’t have made it public”, he told Melbourne’s Herald Sun. “It just promotes their lesbian lifestyle and trying to make it natural where it’s unnatural”.

Nile wants the Prime Minister to have a serious talk with Senator Wong, but suggests that the unmarried Julia Gillard lacks the proper moral authority, as she isn’t in a “traditional relationship” herself.

When Nile and his ilk lecture the rest of us on points of Christian doctrine I tend to leave well alone. But this is the second time in a week he has stepped into areas where he lacks any competence or authority.

First, he pressed Premier Barry O'Farrell to get rid of ethics classes in New South Wales schools, claiming school ethics classes should be based on the teachings of Jesus Christ.

Others have repudiated Nile on this point far more eloquently than I could, including 11-year-old Charlie Fine.

The Office of Senator the Honourable Penny Wong

But when somebody speaks about what is “natural” or even “traditional”, I take notice. As a biologist, I assume they are talking about evolved human behaviour.

Which is why it surprises me how often religious conservatives – most of whom couldn’t say the word “evolution” without choking on their own bile - glibly pass off their prejudices as reflecting the natural order of things.

Gay people, single mums and unmarried parents know what it is to battle this naturalistic bigotry every day. But is there any justifiable basis for it?

Born this way

Fundamentalist homophobes – be they Westboro Baptists or closer to home – relentlessly cast homosexuality as a choice or a pathology.

To these people, being gay is a sad consequence of abuse, growing up in a broken home or with an absent or weak father. In this view, homosexuality is an affliction that can be cured with massive doses of expensive Christianity.

But gay activists, and every gay person with whom I’ve ever discussed the subject, remain convinced there is no choice at all, and that they were born destined to be gay.

The biological and environmental origins of being lesbian, gay, bisexual or transgendered remains a fascinating and complex subject (see Jane McCredie’s Making Girls and Boys, 2011, New South Books, for an excellent recent treatment).

Ignoring nuance for a moment, there’s plenty of evidence from neurobiology and genetics that for a sizeable number of people, being gay is entirely natural.

But, if evolution favours those traits that make an individual reproductively successful, then how could any genes that predispose people to homosexuality have made it through millions of years of human evolution?

To understand what I’m about to explain, it helps to remember that we aren’t talking about a single “gay” gene.

As with most attributes, sexuality is probably influenced by hundreds of genes that find themselves in straight women and men most of the time.

Brendan Zietsch, writing in The Conversation, has [argued](The Conversation](http://theconversation.com/best-of-mates-why-gay-genes-are-a-good-fit-for-darwin-1360) genes that increase the chances of an individual being gay also make straight or bisexual carriers more attractive to members of the opposite sex.

Likewise, genes that improve men’s fitness might, when inherited by a girl, raise the probability that she will be gay. And some genes that make excellent mums might make boys slightly more likely to be gay.

Possibilities such as these are not only plausible – evidence shows they are likely to have kept the human gene pool stacked with genes that shape our varied sexuality.

Beyond the advantages of “gay” genes, though, it’s too easy to forget that for most of history, homophobia kept gay people closeted in unhappy heterosexual unions.

And from there, they passed on their genes.

More about those heterosexual unions

While Nile and his brethren fume about the decline of what they call “traditional relationships” – that is, life-long heterosexual church-sanctioned marriages – we should ask what natural human relationships really look like.

To understand traditional human relationships, we should look at traditional foraging societies – the kind of lifestyle in which our ancestors spent most of the last 2 million years.

Among the remaining foraging societies there’s a fair dose of promiscuity and short-term marriage-like relationships that last between several months and a few years.

Within these societies, though, individuals engage in a remarkably varied range of sexual behaviour from rampant promiscuity to life-long monogamy.

They make the best of the circumstances into which they are born. Thus patterns of relationships, including whether marriages are monogamous, polygynous or polyandrous, depend on the economic circumstances of the day.

Only with the recent rise of farming, no more than 12,000 years ago, did marriage begin to resemble a life-long exclusive rights deal.

With farming came surpluses and thus wealth, and the need to defend that wealth. Families, but especially the men in those families, were well served by a strict division of workload in which the women produced labourers for the farm and the men and pre-reproductive women turned their labour into produce and goods.

Religious institutions, built mostly by men and for men, normalised this way of life, entrenching it in their holy books.

Christopher Hitchens takes the Ten Commandments apart and suggests ten that might better suit the 21st Century. Watch for number five, around 7:00: “Do not condemn people for their inborn nature”.

All of this has been unravelling since the industrial revolution. Those who would use religion as an ancient justification for their preferred social order should realise their holy books are not that old.

Such books reflect the unusual circumstances in a bygone middle-eastern culture and way of life, sporadically updated from time to time over the past two millennia. No holy book reflects the range of natural and normal human behaviour.

Here in the 21st century, men and women share modern work, and accept that families come in all shapes and sizes.

In Australia, we are lucky to have an unmarried woman doing a dignified job as prime minister and a super-competent finance minister who is not only a lesbian but will probably be a sensationally good mother.