What's the reason behind it? If there's a medical reason I would urge them to seek help. If it was something completely unfixable, and for example caused them extreme pain during intercourse, then we wouldn't have sex any more but that doesn't mean sexual activity has to stop altogether.
If they were just completely unwilling to have sex at all, with no good reason for it other than them not wanting to, then the relationship would break down pretty quickly I should imagine. Sex is a big part of being a couple. It's bonding, it shows the other person you find them desirable, it's intimacy, and so on. If my husband just flat out said that sex was from now on going to be a no-go area, I would be hurt beyond words.