After spending a great deal of time discussing and debating the pros and cons of same sex marriage, first of all being assured it wouldn't happen (after all, we already had civil partnerships) then being assured it wouldn't affect anyone, I received a letter from Her Majesty's Government. To be precise, from the office of the then prime minister, David Cameron.

In response to my question on what the government meant by same sex marriage, I was told the government supported redefining marriage because they did not see why two people who loved one another should not get married. It was a trite and superficial soundbite, but as a definition of marriage it was truly dreadful. Immediately I could see that there was no chance of this redefinition (and the subsequent social change) stopping at same sex marriage. However, when I suggested the inevitable consequence would be an increasing acceptance of polyamory and then by default, polygamy I was told not to be ridiculous.

A couple of years later that prediction is being fulfilled. A number of articles in influential newspapers and magazines are now being followed by TV documentaries such as this one broadcast on the BBC this week.

It is one of the most depressing programmes I have seen, not least because it was focused on my country (Scotland) and my city (Dundee). Who knew that there was a Dundee polyamory group? There was a great deal to sadden one's heart in watching this and a great deal to learn about our society.

The English language is being constantly expanded. As with the transgender issue it appears there is a whole new vocabulary to learn. The programme spoke of triads, thruples and V relationships amongst others.

Logic and reason are being butchered so that the language becomes even more meaningless. What else can you can of otherwise intelligent people declaring, 'Although they were fully committed they continued to date other people'? The disconnect from reality is surreal.

It's all about 'me'. One participant declared, 'There are times when monogamy is right for me and other times when poly is right for me.'

A cheap, degrading and selfish view of sex inevitably leads to shallow and ultimately meaningless relationships. One man spoke of sex as just being a 'cardio workout – like going to the gym'.

Mainstream and social media are always likely to be used to promote and advocate increasing aberrations in human behaviour. Having accepted that any form of 'sexuality' and sex is fine they have no real basis for challenging it. The BBC documentary had lovely soft romantic music in the background as we were told that this was just another expression of 'love'. There was no challenge, no cultural analysis and no interviews with those who have been harmed by this practice. It was all positive.

The political parties will follow where the cultural elites lead. In the 2015 election Green Party leader Natalie Bennett stated that they were 'open' to the idea of three-person marriages. In my own church at a political hustings, the man who is now our local MP stated that he also saw polygamy as a possibility. It is after all about 'marriage equality'.

It won't stop with polyamory. Polygamy will follow on. And then what? There is no reason, given the logic of the position that has led us this far, that incest will not be legalized. When I challenged one group on this I was told: 'I don't have a moral objection to incest between two consenting adults either. I would have an objection if there was a chance that a child with a higher risk of genetic abnormalities might be produced. Otherwise I see no reason to intervene.'

What can/should the church do? We should weep as we watch our society regress into the pre-Christian Greco-Roman/pagan view of the world. I certainly wept – because I recognised the people I saw in that programme. Not that I know these particular individuals in person, but I do know plenty like them – young, hip, confused, entertained, materially well-off – and lonely.

There is not much point in waving placards or just protesting, because we would be doing so in terms that our cultural elites largely do not accept. Instead I think the clue is found in the statement that was made more often than any other by the messed up participants in the programme – it's all about love.

The trouble is that they seemed unable to define what love is. But we know. It's not lust. It's not greed. It's not just companionship. It's not selfish. It is 1 Corinthians 13. It is Christ. It is the Son of God who loved me and gave himself for me. God is love. 'Love' is not God.

Our answer to the confused sexual dysfunctionality of our society is to proclaim the love of Christ, to live lives that challenge the cultural narrative and to exemplify God's new kingdom. If our culture is to see a genuine repentance and a renewal of faith in Christ, maybe it should begin with us?

David Robertson is associate director of Solas CPC in Dundee and minister at St Peter's Free Church. Follow him on Twitter @TheWeeFlea