Should a Christian couple live together and have sex if their wedding date is already set? Famed theologian John Piper says no - and those who teach otherwise are "tragically wrong."

On the latest Ask Pastor John podcast, Piper shared his thoughts on the issue in response to a reader who asked, "How would you respond to a group of professing Christian leaders who teach or imply that it's okay for couples to move in together and engage in sex as long as the wedding date is set in stone?"

The DeisringGod.org author first contended that plans are not vows - and it's "not okay" to presume they are the same thing.

"It's mere fleshly pragmatism to treat an intention to get married the same as being married," he said. "They're not the same."

The Bible is clear that any sex outside of marriage is a "prostitution of God's creation", as sexual relations are a "sacred physical, mental, spiritual consummation of awesomely sacred vows made before God."

The "Don't Waste Your Life" author pointed out that in the New Testament, Mary and Joseph - who is described as "just a man" - were not intimate before marriage.

"Mary and Joseph were chaste," he said. "They were not having sex. This was part of what it meant that Joseph was a just man. You're not a just man if you cave in to the worldly pragmatism of just saving money on rent and jumping in bed together. That's not a just man. That's a weak man with little biblical principle."

Ultimately, God didn't design sex to be just a "relief valve for desire or a mere pleasure with a devoted lover."

"The Bible does not recognize a legitimate use of sexual relations except as an expression of the covenant of marriage - no matter how crazy our modern world has become and almost all media and entertainment have shown," he emphasized.

Rather, sex was created to be "the consummation of the sacred covenant of God in marriage," Piper said.

Those who wait for marriage, then, could "set a beautiful example to their peers, to children that are watching, and to the teenagers around them that are all watching, knowing what they do shows that sexual relations belong in marriage".

"They will want to witness, with their lives, that God created this beautiful gift and placed it precisely where he knew it should be," he concluded. "The most satisfying, most fruitful, most beautiful, most God-honoring place is in marriage. Therefore, Christians do not sleep together before their wedding night."

According to the "2014 State of Dating in America" report published by Christian Mingle and JDate, 61 percent of Christians said they would have sex before marriage. Fifty-six percent said that it's appropriate to move in with someone after dating for a time between six months and two years.

Responding to the data, Peter Sprigg, senior fellow for policy studies at the Family Research Council, told The Christian Post that far too many Christians today are more influenced by the culture than they are by the teachings of scripture or the church.

He also suggested "there may be a weakness on the part of churches" that explains the gap between sexual behavior and biblical standards. He described "a vicious circle," where a pastor welcomes people regardless of their past sins, and then fails to preach biblical morality due to a fear of being considered too harsh.

"I would encourage pastors to speak bluntly and boldly about sexuality and a biblical view of sexuality and marriage," Sprigg said, and urged religious leaders to "not shy away from challenging the values of the culture."