According to Tim Lane, this sort of statement is all wrong and is, in fact, dangerous.
Lane argues that there is no place for sex outside of marriage in any circumstance. In fact, he argues that couples who are not married should not even do anything that might lead them to want to have sex. Clearly, he does not agree with the idea you cite in your question.
Lane argues that God does not want us to have sex outside of marriage. He argues that sex is simply too important to be engaged in outside of marriage. No matter how committed the two people say they are, they are not permanently bound together if they are not married. They have not made the sort of commitment that is necessary in order to have sex.
Sex is one form of union with another person, Lane says. But it cannot properly be engaged in unless it is part of a complete union with that person. Complete union can only exist if the couple is married.
Therefore, a Christian should not accept this statement in any way.