In your book, “Not ‘Just Friends’,” you say more “good people in good marriages” have affairs.

Traditionally, male affairs were just about sex. But those affairs are increasingly being replaced by passionate love affairs, with men getting more deeply emotionally involved and women getting more sexually involved.

How do modern affairs start?

Some start on the Internet. Say a man meets an outgoing woman in a chat room. As time goes on, they begin to share more about themselves and their fantasies. Soon they’re switching screens when their spouse walks in on them.

What’s the warning sign that a friendship is becoming an affair?

When a person starts sharing their marital dissatisfaction with someone other than their spouse. The friend now has a window into the marriage, and the spouse has a wall [against] the friendship. In a healthy marriage, we open windows to each other and keep walls to the outside world.

Are modern affairs more threatening to marriages?

Yes. When a sexual relationship is preceded by emotional intimacy and bonding, it’s a much deeper relationship than one that begins with just sex.

What is the “prevention myth”?

People think having a good marriage will protect them. But in one study I did, 56 percent of the unfaithful men said they were happily married.

One of your motivations for writing this book seems to be your concern that too much bad advice is being given to people dealing with infidelity.

That’s true. It’s a myth that the person cheating must not be getting enough sex at home. Or that when an affair is over, it’s better not to talk about it, because that’s too painful.

After an affair, how do you know if the marriage is worth saving?

I don’t think anyone knows initially, but the worst thing you can do is make a hasty decision. You may think your marriage is on its last chapter, but you may just be in the middle of your story together.