since sex is like 90% of a relationship I'd have to say yes. If your sex life sux, so will the relationship... if it's good, the relationship has a chance... that's just a fact of life.
I don't care what anyone else says... reality is if sex with your partner is terrible... it'll never ever ever last. The relationship will eventually be over in due time... period. So maybe sex is 100% of a relationship <shrug>