We cannot use what was done in times before Christ as an excuse: Acts 17:30 "And the times of this ignorance God winked at; but now commandeth all men every where to repent.."
This is not really a mystery if one understands that the core thought behind all of the various ways pornea is expressed is idolatry.
Colossians 3:5 "Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry:"
Romans 6:12 "Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof."
Every Christian needs to see that it is idolatry to chase after their lusts and not wholly dedicate their bodies as sacred vessels to be used as holy instruments glorifying God.
With that in mind, one can see how easy it would be to rationalize sexual activity between consenting unmarried adults as a guise for having an avenue to step around the need to have complete control of our lust for sex.
Disguising it as OK because, "we are in love and are consenting adults", is a facade of self-deception. No one can say for certain that a legitimate marriage will occur until it does. And if everybody used that excuse there would be many for whom that marriage would not materialize.
Also, simple logic also tells us that to do this sets an unloving example for others to follow which might easily land many of those who follow our example into unwanted pregnancies, sexual diseases, injured emotions which can give rise to crazy things like as the Jodi Arias case highlights, not to mention distracting their attention from off God which even in a small way is idolatry. All of this is so, whether we our self escape those consequences or not.
If you would not recommend your child to have sex by consent, (and you know why you wouldn't as it is dangerous in many ways), then it is a clear violation of love and a hypocrisy toward your child or toward other peoples children, knowing they like to grow up so fast, to have set such an example for them. We must set the example in doing what is proper and good and that requires we demonstrate complete mastery over our lusts.
Back to what Paul spoke of at 1 Corinthians 7:31-36, see that it is about our not being distracted from God. Whatever distracts us has become as an idol between us and God. God has allowed us the avenue of marriage which distracts us somewhat and he will not take kindly to our grasping at more than that as the cause of our lessened attention toward him. And that is not selfish. He is our source of life and must be looked to in all seriousness with out distraction if we would be able to walk after his spirit not side-stepping.
if we still operated under mosaic covenant then I would be totally for all of that