If you think that the person might respect the Bible, you could refer to a passage such as 1 Corinthians 6: 18, which says that the person who has premarital sex is sinning against, or harming, his or her own body.
Fa o akanya gore motho yoo a ka tswa a tlotla Baebele, o ka nna wa umaka ditemana tse di jaaka 1 Bakorintha 6: 18, e e reng motho yo o tlhakanelang dikobo pele ga lenyalo o leofela mmele wa gagwe kgotsa o a o gobatsa.