“how early/late in a relationship (as things approach marriage) should one person tell the other about previous romantic experiences/sexual sin?”
Regarding the question of ‘when’ you disclose certain information, it depends on the level of emotional intimacy in the relationship; the more intimacy more must be unveiled. First off, I believe everything must be disclosed between future spouses at some point. I don’t think hiding sin does any good, as it only shows that sin still has the power of shame over your life. Jesus died to wash you clean. He bore that sin on the cross to announce that it is as far from you as west is from the east. There is no condemnation now to those who are in Christ and every wicked thing you did is only another voice in a great symphony showcasing God’s redemption through his son Jesus. Yet your and their pasts will become a part of the new union and your former struggles or issues may influence both of you so they ought not be hidden. Of course there ought be not glamorizing or wishful reminiscence of sin, but instead a joyful exuberance that Jesus saved you from it.
I think the biggest thing you want to be watchful for is your potential partners attitude when he or she does disclose things from his/her past. Do they wave it off as nothing important or as just a part of growing up? Or are they genuinely repentant and honestly wish it had not happened.
DON’T IGNORE COUNSELING
First off, I do recommend you start by taking some good premarital counseling classes. Yes, not everyone needs them. Yes, not everyone has taken them. Yes, marriages have survived without classes. Yes, people have had happy marriages without counseling. Not everyone needs to take drivers education, however, it’s so much better to learn to drive with professional education. Today’s shifting cultural atmosphere has created very false ideas of marriage, romance, and love. It is much easier to handle these issues with a mentor or someone to give sound biblical council. Even though it wasn’t called premarital counseling, the giving of advice was always practiced throughout history. Advice is good, unfortunately not everyone is qualified to give it, so therefore a good godly pastoral counseling program will do much benefit for you. Look at your local church or other that can help you at such a time. For example Mars hill offers good premarital counseling that is nearly free (I think you only need to buy a few books).
SOME DETAILS SHOULD BE KNOWN BEFORE DATING BEGINS
Dating/Courting should be public
Even though this isn’t often practiced in some parts of the Slavic culture, it is best for everyone if dating is public (and therefore the couple is publically accountable before everyone). When I started dating my wife, we kept the details of our dating private from those around us. This is somewhat because of the subculture of my parents which assumes that every time a boy talks to a girl he is proposing marriage, and the fact that my father is a pastor. Put the both together and you have a recipe for becoming very popular very fast. Fortunately our parents and families knew which kept us accountable. Unfortunately many people date completely out of the public eye, this is neither smart nor safe. Ideally you would be dating under the careful guidance of and friendship with your Christian community. Ideally dating should take the form of a guy getting to know a girl as a friend, without having promised each other the rest of their lives.
Dating history should be fearlessly disclosed
If dating is public and accountable before reasonable parents, there should be no reason to hide it. If your dating consted of being friends with a guy/girl and getting to know their heart in order to discern whether God wanted you to pursue life together, there is no shame. There is no reason to hide the fact that you spent time with someone of the opposite sex before agreeing you were not right for each other. This can be disclosed very early in any relationship. Generally most of your history that does not involve sin ought to be open to each other, and very early. There may be some fear about divulging this kind of information, and you may think “what if they will stop liking me?” If the other person will leave you for it, then trust me, it is far better to reveal it as soon as possible and get it over with before serious heartbreak.
SIN SHOULD BE DISCLOSED WHEN MARRIAGE IS SERIOUSLY CONSIDERED
In general, intimacy is built as two people share more secrets about themselves finally progressing to marriage where they are “one flesh” and can share every desire or secret with each other, without the fear of judgment, rejection, or neglect. It may be different for each individual couple and the exact route they take to marriage, but overall I feel that its best to disclose these things before or about the same time a marriage proposal is done. If you are planning on taking premarital counseling you will end up having to work this through anyway, though I do recommend not saving any surprises for counseling.
Know your prospective spouse
First off, you need to know your likely future spouse very well. In fact, you need to know them well enough to answer, before you tell them, what their stance is. Do you think they most certainly reject you if you are not a virgin and refuse to marry you? Then it may be wise to tell them you are not virgin without the details and see if they feel the pain with you but forgive or if they start to back out and treat you as unworthy of them. If you think this will be their reaction then you really would be wise to tell them before a proposal is made.
Be regretful of sin when you tell
Once you realize the time has come, don’t wave it off as something unimportant, don’t say “oh by the way, this one time I also did ______ with this guy as well, but it was nothing really.” Sin is an occasion to grieve and be remorseful, and sexual sin is no exception. If you didn’t feel sexual sin was important enough to repent of and don’t feel remorse when you tell your “future spouse” you should go return the engagement ring and buy yourself a one way ticked to Antarctica.
Be forgiving if you are told
If you start revealing you sin, it’s likely your future spouse may have some of his/her own to shamefully bring before you. However, whether or not you sinned as well, if you believe the Bible, you are called to forgive and hold no grudge. It’s a radical difference from “common sense.” A few generations ago, a woman who exposed her former sexual sin, even if it was repented of would surely be slandered and ridiculed everywhere by popular culture. Today’s culture won’t do it not because they forgive, but because they don’t think it’s wrong in the first place. You are called to neither shame for forgiven sins, nor to act like sin is nothing, you are called to forgive that sin which Jesus bore upon the cross. Don’t compare yourself and how ‘good’ you are, for surely in other spheres of your life you have sinned much.