US church leader and theologian John Piper has addressed a controversial topic in the latest 'Ask Pastor John' segment on his blog – would he marry a couple who have chosen to live together before they wed?

Those who do not support pre-marital unions suggest that by agreeing to marry a co-habiting couple, a pastor endorses sin. Some even suggest that it is no different to marrying a gay or lesbian couple, which remains a hotly-debated topic within the Church.

Piper – a staunch opponent of homosexual marriage - however, differentiates the two. "I would marry them in certain circumstances," he reveals of co-habiting heterosexual couples.

The pastor notes his conviction that "sexual relations outside marriage is sin", but that "it is not unforgiveable". Piper shares that he would suggest that such a couple "repent and bear the fruit...And that fruit would be move out and stop living together until you are married. Now, if they refuse I tell them: 'No, I won't do the wedding'.

"But here is the other situation. If they see the wrong of what they are doing and repent and bear the fruit of purity and public display of the lordship of Jesus in their lives, then I would move forward with their wedding plan, all other things being in proper order," he says.

"They are really serious about repenting of their sin. They are sorry for what they have done. They are turning to God for forgiveness through Jesus. And that bears a beautiful testimony of the grace of God in their lives. In that case, yeah, I will follow through with the wedding."

Piper adds that the "failure to offer pure and virgin bodies to each other at the altar of marriage" brings him "a great sadness", but that "purity on the other side of sin is possible through the justifying and sanctifying work of Christ".

"That is what I want these couples who have sinned to embrace," he says.

"The fact that they in their past have the sin of fornication...doesn't diminish the possibility of purity and holiness in the present and in the future."

Piper then goes on to distinguish between this and the marriage of same-sex couples, noting that true repentance results in transformation and change.

"If two men had been sleeping together in a same sex relationship and came to me wanting to marry them and said they were sorry for the sin of sleeping together before they were married, I would willingly help them receive forgiveness and trust Jesus and find repentance and move forward in holiness," he shares.

However, "If they then looked at 1 Corinthians 6:9-10 which says men who practice homosexuality will not inherit the kingdom of God...and still wanted to move forward into a relationship I would say: 'No, because now you are moving back into the very same sin that you just moved out of.'"

Gay marriage, Piper argues, "can't be used as a warrant in order to justify homosexual behaviour" because there is no precedent for it in scripture.

"Paul says in 1 Corinthians 6:9-10: 'Men who practice homosexuality...', he doesn't say: 'Except in the case of marriage'. There is no such thing as homosexual marriage. It doesn't exist. It is a mirage in our culture.

"So I want people to repent of the sin they have done. God holds out hope for all sinners who will turn away from the sins of the past, embrace Christ as their righteousness and their forgiveness and move forward into lives of holiness."