Lille,
Marriage encounter is about communication and sharing, not about sex. Yes, to be most effective, both partners have to participate fully, but on the other hand, if you expose yourself by disclosing fully, just that act will help to get your spouse to open up. In my case, I went into the weekend viewing it almost as a last chance so i had nothing to lose by just opening up totally. My W was VERY reluctant (if you look back to my threads you'll see that), to the point of almost bagging out as we pulled into the hotel. Had it not been for her best friend taking the kids and insisting she go, we would have never made it there. By the end of the weekend, she was opening up too. Because of her reluctance, I really didn't expect much either, but I decided to put everything I had into it in order to have the best chance of success. I can't say enough good things about how it affected us.

Wildebub,
Marriage encounter is different because it is not counseling. It is a series of exercises, that you and your spouse participate in privately in your room, designed to get you talking intimately. I'd say a minimum requirment is that both partners have to be committed to the relationship (and I'm sure all of you are, or you wouldn't be here). This is much different than counselling, as the purpose is not to analyze and fix the problems, rather it is to learn to communicate your feelings. It is amazing what just knowing what your spouse is feeling because of your actions can do for getting you both to move closer together.

What can you lose by going? Worst case, you are out a weekend of your time. I suppose you could discover that you despise each other and that you are better off splitting, but even in that case I suppose the weekend makes you better off. You don't even have to pay for the weekend. They tell you what it costs to host a couple for the weekend, and ask you to donate what you feel you can, but there is no obligation to pay anything.