The first thing to say is that the bible's sexual ethic is different to the world's sexual ethic. And it always has been. This might sound too basic to mention, yet the point is commonly forgotten. Egyptian and Assyrian views of sex were markedly different from Israel's. As for the Greco-Roman culture surrounding the New Testament church... what's the saying? "A woman for necessity, a boy for pleasure and a goat for ecstasy!"
Both Christians and non-Christians need to realise that the bible's ethics were never the ethics of the surrounding culture. Therefore Christians ought not to hearken back to some golden age when the bible's norms were upheld by the culture. Neither should non-Christians insist that "Christians move on from their conservatism and embrace a new golden age." The truth is that the bible never enshrined the culture's sexual ethic - it always stood apart from it.
This leads to another basic observation... secularists need to recognize that they too have a sexual ethic. They are not champions of liberation - except in the most limited sense. They are simply trying to impose a different sexual ethic and therefore to define a different set of sinners.
The next point is the explosive one, but it needs saying in order to blow apart some suffocating assumptions: Jesus is utterly anti-heterosexual. It's difficult to think of anyone as anti-heterosexual as Jesus.
I mean really, can you imagine Jesus in the sermon on the mount turning to his disciples and saying “Let your sexual desire be unto the multitude of women.” If you can imagine that sentiment on the lips of Jesus, you don’t know Jesus! Christians are not – or at least should not be pro-heterosexual. Lust is lust and never a positive marker of identity - no matter which cross-section of potential sexual partners are in view!
These modern taxonomies of sexuality are so limited, so unruly, so new, so western, so 21st century. We struggle to apply them to other 21st century westerners, let alone other parts of the world, let alone other parts of history. If you try to use our modern categorisations and apply them beyond our tiny blip in time and space you're in for trouble. If you want to actually listen to Jesus' teaching on sex you'll need to forget everything you think you know about "modern liberal" notions and "out-dated conservative" notions. Because Jesus' teaching is something else...
Jesus’ view of sex is crazy and it’s beautiful. Same as everything else. "Turn the other cheek? Go the extra mile? Love your enemies?" Crazy! Impractical! Totally unrealistic! But beautiful! Let me explain...
Jesus only really said three things about sex, but on these three foundations you have a crazily beautiful / beautifully crazy view of sex. In Matthew 19 He quotes approvingly from Genesis 1: “In the beginning the Creator made them male and female.” Then Jesus quotes from Genesis 2 “For this reason a man shall leave his father and mother and be joined to his wife and the two shall become one flesh. What God has joined together let not man put asunder.” Combine these two truths and what do you have? You have humanity created as interlocking opposites who become "one flesh" when man and woman come together in an exclusive permanent marriage bond. That’s his teaching on sex and marriage. And to that He adds, in Matthew 5, that sex is not for any other context. Not even in your thought life. Don’t even think about sex outside marriage. That’s Jesus’ sexual ethic. It’s crazy, but it’s beautiful.
Because, according to Jesus, when you have sex with someone you are saying “I give myself to you utterly, exclusively, irreversibly and unconditionally for life.” It’s the most romantic view of sex the world has ever seen.
This is sex as a Ferrari. If I owned a Ferrari, you could not drive it. Only if your name was Scrivener could you get behind the wheel. But if I owned a beat-up old Lada – anyone could drive it. The Christian view of sex is a Ferrari. The modern view is a Lada.
But for that reason, this sexual ethic is for the followers of Jesus. Explicitly the bible tells Christians not to bother non-Christians about their sexual ethics. 1 Corinthians 5 tells Christians not to worry about what people are doing outside the church. God can worry about them, we’re meant to only worry about ourselves. This point will be controversial among Christians but I suggest that, in line with the first truth outlined, we address ourselves with the ethics and the world with the gospel.
Which means that the question for the non-Christian is not "Can I live under this sexual regime"? The question is, What do I make of Jesus? If He rose from the dead as Lord of the world, then maybe He knows a thing or two about sex. And if I come to Jesus - not as hetero-sexual, homo-sexual or bi-sexual but simply as a sinner - then there's a place at His table equal to every other sinner. And though I fail at His crazy-beautiful life in a thousand ways, He knows how to lead me, step by step, into greater and greater freedom from sexual slaveries as well as the other really dangerous sins - like greed, unforgiveness and moral self-righteousness!
For another approach, here's an older post on the subject...