Continuing my response to Nathan’s question in the comments:
Nathan asks (Part 2),
Is adultery a “one-time” sin that is committed on the day of the marriage ceremony, or is it an ongoing sin? Surely we would teach that individuals in homosexual or polygamous relationships would have to end those relationships in order to be faithful. We wouldn’t teach that baptism would forgive past sin AND sanctify the continuation of a sinful relationship. Yet is that not what we do when it comes to adultery?
Apples and oranges.
The passages dealing with homosexuality aren’t speaking of gay marriage but the actual sex act — which is sinful per se.
Sex between a man and woman married to each other is not sinful. And in both Matt 5 and Matt 19, Jesus never says that the second marriage is void. Rather, he refers to it as “marriage.” Hence, it’s a marriage.
He is concerned that in some circumstances marriage can be a violation of a prior marriage covenant. Paul tells us that if we divorce, we should try to reconcile (1 Cor 7:10ff). If you remarry, you can’t reconcile. If you remarry, you can’t honor the original marriage covenant — ever. Deu 24 forbids it (making the notion of divorcing the second wife to return to the first wife as essential to one’s salvation absurd; it’s forbidden).
Now, the pattern of the Sermon on the Mount in Matt 5 is for Jesus to point out an abuse of the Torah and then to show the real heart of the Torah. The abuse of the Torah was to seek out a second wife while still married to the first. (This presumes monogamy, by the way. In a polygamous society, the husband would be well within his rights to lust after a single woman not his wife.) But Jesus, based on Gen 2 (see Matt 19), is expecting monogamy. And monogamy means not even looking at the second woman lustfully.
Divorces occur for multiple reasons, but one common reason is lustful looking and a desire to “trade up.” And this is to objectivize one’s first spouse (the sin of objectivizing other people is a recurring theme of the Sermon on the Mount) and to be unfaithful to her. Divorcing a wife to gain a new wife is adultery against the first wife as a breach of the marital covenant. It’s point in time sin. And it’s sin.
But sin doesn’t mean it didn’t happen. The marriage is quite real, as is the divorce — even where both are sinful. Being sinful and being void are two very different things. Sin is about our relationship with God. Voidness is a question of law. The Sermon on the Mount is about moving from law to right relationship. It’s not legislation; it’s examples of how to correctly read Torah in light of the nature of Jesus (and God). And faithfulness to one’s covenants is at the heart of the nature of Jesus (and God). Therefore, violating the marriage covenant is sinful, and the fact that divorce is permitted by Moses does not mean that divorce is always consistent with one’s obligations of faithfulness.
The entirety of the Prophets and the Torah can be told as a marriage story of God marrying Israel, and Israel being an unfaithful wife. As we strive to become like God (Matt 5:44-48), we must learn faithfulness to our spouses.
So unfaithfulness is a sin, and breaking a marriage is a sin. But the marriage is still broken.
To read this as a text about marriages that aren’t marriages and divorces that aren’t divorces is to read like a lawyer (which I am one) and not a very good one. We’re looking for laws at time when we’re being freed from law (Paul still says what Paul says).
So homosexual acts are forbidden, and marriage and baptism do not change the fact that a homosexual act is a homosexual act. Incest is forbidden, and marriage and baptism do not change the fact that she’s your sister.
For second marriages, the baptism doesn’t cure the void remarriage. Rather, it forgives the sin that the divorce might have been and that the remarriage might have been. Sex in a heterosexual marriage is not sin, and so there’s nothing about sex with your wife to forgive. But the making of the marriage may well have been sin, and that sin needs to be forgiven.
Just so, if the sinful remarriage occurs after baptism, forgiveness is available — but only for those who repent. And you repent of being unfaithful to your spouse by being faithful to your spouse. But your spouse is the person you’re married to, not the person you used to be married to.