How to Counsel Couples Through Past Sexual Sin
Remember when doing pre-marital counseling with young couples regarding sex involved a brief warning about temptation and supplementing their education about the birds and the bees that their staid or embarrassed parents had neglected to mention?
Yeah, me neither.
If you’re reading this article, then you’re likely dealing with couples where at least one person has committed repeated and perhaps grievous sexual sin. Now, they’re trying to navigate through this reality toward a godly, healthy marriage in which the “trust bank” has been desperately depleted and needs to be restored.
Practically, let’s talk about when and how pastors can guide dating or engaged couples through these difficult conversations.
There’s no hard-line answer here, but my best recommendation is that past sexual sin should be discussed in that awkward stage in which the relationship is going well and likely headed toward marriage but before the couple is formally engaged (for a cure to insomnia, see my articles [here] and [here] on stages of a pre-marital relationship). In our culture, engagement is considered a serious commitment that requires a bit of weeping and gnashing of teeth to break, though it happens more often than you think. Given that fact, people should have an opportunity to know, before making a commitment to marry someone, that his or her potential spouse has had sex with numerous people or until last week was in the midst of an active addiction to porn. I’ll talk about the biblical responses to such information below, but it seems wise that the “committer” of sexual sin should confess before the prospective spouse has reached the theoretical and cultural point of no return.
Having said that, we shouldn’t encourage couples to talk about past sexual sin too early. Revealing intimate details of one’s past sin early in a relationship is generally not a good idea because (1) it tends to create an inappropriate level of intimacy for the early stages of a relationship, and (2) it tends to put an unfair burden on a new relationship by asking people to deal with really difficult things about their partner’s past before they really know each other’s current character and walk with Christ.
First, past sexual sin should be talked about in general terms. Confessing the mere fact of sexual sin with other partners or a past struggle with pornography may well be enough. Reasonable follow-up questions might discuss the number of sexual partners, whether they’ve experienced same-sex attraction, or the timing and level of victory over porn. Beyond that, details are usually good for no one’s soul and generally unhelpful unless they’re genuinely relevant to the wisdom of a marriage decision. It’s also not a topic to be repeatedly dwelt upon if that can be avoided.
So here’s my baseline advice: past or present sexual sin should be discussed in a single conversation in which both people confess what they need to. Perhaps this is followed up by additional conversations as people need to process what they’ve learned.
Past sexual sin should be confessed with humility, empathy, and likely some measure of sadness or regret, but not guilt or shame, inasmuch as the Lord Jesus Christ has accounted for such sins on the cross. Still, brothers and sisters dealing with sexual sin—and revealing it to someone they hope will love them—may be wracked with guilt and shame that need not be a part of the abundant life in Christ.
On the other side of the equation, a potential spouse ought to hear confession of past sexual sin with sadness, regret, even frustration if that’s where the heart is—but ultimately with an attitude of grace.
To be clear, the wise and godly response will not always be to move forward with the relationship. Being informed of a potential spouse’s prior addiction to child pornography, for instance, comes to mind as an issue that may genuinely affect the wisdom of a marriage decision. Still, in dealing with past sin, the godly response should most often be a willingness to proceed with the relationship if, on balance, other factors already point in that direction. 
At the same time, someone who hears such a confession from a potential spouse—especially if their own sexual sin has been comparatively less profound—can struggle with sadness, anger, bitterness, fear, and self-righteousness.
So how do we advise men and women in one or both of these positions?
As I suggested above, fundamentally this is an issue of showing grace to a fellow sinner forgiven in Christ. In fact, most of the same basic scriptural principles speak to both the offender and the offended. Though sexual sin (like all sin) is most fundamentally committed against God, it’s also a sin against any other person involved and the sinner’s future spouse, so it’s perfectly understandable for a potential spouse to respond with feelings of hurt and sadness.
But if the sinner is now in Christ, then he or she is “a new creation. The old has passed away; behold, the new has come” (2 Cor 5:17). If a sinning believer has confessed his or her past sins to God as his child, then God has not only forgiven them but has “cleansed [him] from all unrighteousness” ( 1 John 1:9); he has thrown those sins into the sea to remember them no more. God doesn’t just forgive; he forgets. When he looks at his children, he delights in us because he sees his perfect Son. And so he calls us to view our brothers and sisters in Christ in the same way. This is precisely why Jesus himself has harsh words and a stark warning for those who are forgiven but cannot forgive (Matthew 18:21–35).
As pastors, we need to remind young couples not only that we are all sinners (Romans 3:23), but also that we are all sexual sinners. Even if a man or woman has not sinned sexually with another person, pornography use, masturbation, and lustful thoughts all count against them, ruining any perceived perfection. We’re all fallen sexually.
But there is grace and healing in the gospel. Encourage young brothers and sisters both to rest in the grace God has shown them in Christ and to show their potential spouses that same grace. 
Finally, a little practical encouragement: The emotional, spiritual, and sexual intimacy that flourishes in a loving, godly marriage often goes a long way toward healing past hurts. It has a way of crowding out feelings connected to past sin.
So remind your sheep that God ordained marriage; that he is for sexually healthy marriages; and that every godly, loving, and gracious marriage—all of which involve two sexual sinners—mirrors the gospel and glorifies God.
 How to deal with a current addiction to porn or ongoing sexual sin is a topic for another day.
 This may go without saying, but as you have these conversations, keep an eye out for those of your sheep with deeper issues surrounding sexuality or deeper issues of emotional and mental health more generally. Especially around the issues of past sexual abuse or trauma, longer-term loving care and counseling likely will be called for.