Does a good relationship have to have good sex?

I have seen so many relationships go down the drain these days on just the fact that one partner is not satisfying the other.No Ifs, Ands, or Butts And at this I am almost stumped....does a good relationship have to have good sex to stay a "happy couple", or can there be bad sex or "dry spells". Or at least have a real chance to talk it out or work on it. Is there ever a chance in those relationships gone bad to have room for improvement, or do all us girls out there just find good sex as the one thing we pose as the "tip of the iceberg" for our relationships? I found that talking it out, or that in my friends relationships they try new things, or try not to focus on the physical attraction and more on the mental and emotional part of the relationship. Can we live happily in a relationship with moderately good sex and be happy or has good sex become a necessity in all good relationships and even marriages?