1 Corinthians 6:18

"Flee from sexual immorality. All other sins a man commits are outside his body. but he who sins sexually sins against his own body."

Galatians 5:19

"The acts of sinful nature are obvious: sexually immorality, impurity and debauchery..." etc, etc,

1 Thessalonians 4:3

"It is Gods will that you should be sanctified: that you should avoid sexual immorality"

(the notes in my NIV study bible say of 1 Thes 4:3 "In the first century moral standards were generally very low, and chasity was regarded as an unreasonable restriction."

So it all comes down to guessing what Paul meant by sexual immorality then?

Given that the churches he was writing to had a climate where pre-martial sex was probably the lowest rung of the sexual immorality ladder (apparently the Greek verb "to Corinthianise" came to mean "to practice sexual immorality"), I find it odd that people think they can guess what Paul meant by that single word and therefore outlaw any form of sex outside of marriage.