The First Epistle to the Corinthians states "Flee from sexual immorality" and lists adulterers and "those who are sexually immoral"/practicing-fornicators in a list of "wrongdoers who... The Apostolic Decree of the Council of Jerusalem also includes a prohibition of fornication.Throughout history, most theologians have argued that any and all forms of premarital sex are immoral.
Not that we're blaming them, these couples are aware of the choices they're making and the consequences that come with these decisions.
The fact that there's actually a market for married people in online dating baffles us a little.
An historical example is the medieval English monastic, John Baconthorpe.
A more contemporary example is the modern-day theologian Lee Gatiss who argues that premarital sex is immoral based on scripture.
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