Just because something is true doesn't mean it needs to be said.  Everything that crosses your mind shouldn't necessarily come out of your mouth.  Honesty is sometimes a cross between stupidity and cruelty, and when it comes to keeping secrets from your spouse, honesty is not always the best policy. 

The most successful long-term relationships are based on very strong emotional and physical connections.  However, intimacy is not necessarily equated with absolute and complete honesty. 

There are obviously some things you ought to be honest about:

Debt.  I'm amazed at how many times I talk to somebody who wasn't told that their spouse-to-be had college loans, credit card debt, and no money in the bank until after they were married.  A lot of times people don't find out that their new spouse is in debt until they see their credit when they're applying for a mortgage.  Hiding bad money skills leads to feelings of betrayal and mistrust that can sometimes never be overcome.  A lot of people walk away from marriages in which their spouse put the family in debt due to their spending or unwise financial schemes.

Eating disorders and substance abuse.  If you have an eating disorder or a substance abuse problem, you have to disclose it in advance.  Ongoing substance abuse or addiction will almost always interfere with intimacy.   Why?  Because something else is more important than your spouse.  Unless it's dealt with, addiction will destroy just about every relationship.  That's the truth.

Past illegal activities.  It's best to reveal and explain past crimes and jail time because not only are they available on public record, but keeping them hidden only fills you with enormous guilt.  And if you're exposed, it will scare everybody into thinking that the behavior could be repeated.

Molestation.  There was a period of a couple days where it seemed like just about every woman who called in to my show had been molested when she was younger and had no interest in having sex with her husband.  I put a question up on my website asking women who had been molested if they thought female molestation victims should ever get married.  When reading the responses, I was shocked to find that most of them said, "No."  That blew my mind.  Quite frankly, I didn't expect that would be the answer.  Considering this, I think that if a molestation is clearly still part of your life today, you need to tell your spouse-to-be.  Generally when a woman is a victim of molestation, the molester never gets exposed and the woman feels residual fear and anger.  They feel bitter about justice not being served and get mad at their parents and whoever else failed to protect them.  However, it's their spouse who ends up getting the brunt of it because that's the one area where the victim has power.  Withholding sex from their spouse, for example, is a way they maintain that power.  It's not really all that complicated.  Past molestation should be revealed because it can lead to a very painful marriage if left concealed.

Lack of sex drive.  There are a lot of women who keep their disinterest in sex a secret and fake their orgasms.  Some guys keep their Viagra in a hiding place.  Although we live in a culture that is erotic and pornographic, we don't seem to be able to talk about intimacies with the people we're closest with.  Every day I get a call from some man or woman upset because they have misinterpreted their spouse's physical disinterest as a lack of love and caring.  The first thing I always recommend for couples to do in this situation is for them to each get a complete physical.  Check everything out.  Examine your hormone levels because they have a lot to do with your sex drive.  Next, take a look at your schedules and lifestyle.  What is it that's making you tired or preventing you from being playful and affectionate?  It's sometimes advantageous to talk to an independent third party like a therapist or a religious person who has counseling experience.  

Because of the above concerns, I think it's important that you go through six months of premarital counseling with your spouse-to-be so these secrets get uncovered.

However, not everything needs to be shared with your spouse.  Intimacy and complete openness are not one and the same.  You need to share your vulnerabilities with each other, but you also have to be sensitive to the consequences that sharing brings. 

Affairs.  I've had a lot of people call in to my show saying, "Oh my gosh, I had a half-hour fling, but I know it was stupid and wrong.  I understand why I did it, and I take full responsibility for my actions.  What should I do?  I don't want to lose everything I have."  My response is, "Well, get yourself tested to make sure you didn't contract a disease, and then keep your mouth shut until the day after you're dead."  A lot of times, the spouse who had the affair wants to unburden themselves so they can feel better.  However, if they do, they are only going to destroy the trust in the marriage forever.  If you're truly remorseful and you're not going to repeat the stupid mistake, it's best to just get on with life.

Not everything that can be said should be said.  If you have fantasies about someone, don't tell your spouse.  You're only going to make him or her feel inadequate.

Just about everybody has a list of stuff they've done when they were younger that they're not proud of and they've learned from.  If a past behavior is clearly no longer part of your present (e.g. speeding tickets, fist fights, etc.), file it in the "private" cabinet and keep it to yourself.  Keep it safely between you and you.  If it's seriously no longer part of your life, leave it alone because you'll be judged by what is no longer true.