From First Corinthians by George T. Montague, SM, commenting on First Corinthians 6:18:
The only possible conclusion is to flee fornication. The NAB translation, Avoid immorality, though admissible, loses the strength that Paul intends here: shun, escape from, flee. Porneia may be translated “immorality,” but the context suggests more specifically sexual immorality or fornication. Following Saint Thomas Aquinas, spiritual writers have advised that while other vices call for a tactic of resistance, fornication calls for the tactic of flight, lest passion be enkindled by toying with the occasion.