I don't claim to be an expert, but it seems to me that barring a health issue that keeps you from having sex, there ought to be sex in a marriage, not necessarily a relationship.
Sex is a way of verifying and expressing love, or at least attraction, to your partner. If you ain't doin' it because one or both doesn't want to, that seems to me that either the attraction or the love is missing, and that kind of rejection from their own spouse over a long period of time can seriously damage a person's self-image. If you truly love your partner, you wouldn't intentionally or knowingly subject them to that kind of psychological harm.