The question is, is there really a Bible rule explicitly prohibiting sex without marriage? Because this would be the only possible reason why loosening those rules would even come up in a discussion.
Great point. I'm not sure there is. We have plenty of verses telling us who's "nakedness" cannot be laid bear, but none of them involve two single people. OTOH, there are verses that put value on virginity. This, however, is not the same as condemning premarital sex.
There is lots of good reading out there on what porneia actually means given language and cultural rules extemporaneous to Paul's writing. These are different from how religious institutions have interpreted things for current language.